sign in sign up
<<
Home , Plasmid

2, 466-473 (1979)

Nomenclature of Transposable Elements in Prokaryotesl

A. CAMPBELL P. STARLINGER Department of Biological Sciences, Stanford lnstitut fur Genetik, Universitiit zu Koln, University, Stanford California 94305 D-500 Koln, West Germany

D. E. BERG D. BOTSTEIN Department of Microbiology and Immunology, Department of , Massachusetts Washington University, St. Louis, Institute of Technology, Cambridge, Missouri 63110 Massachusetts 02139

E. M. LEDERBERG R. P. NOVICK Department of Medical Microbiology, Public Health Research Institute of the City of Stanford University School of New York, New York, New York 10016 Medicine, Stanford, California 94305

AND

W. SZYBALSKI McArdle Laboratory for Cancer Research, University of Wisconsin, Madison, Wisconsin 53706 ReceivedMarch 21, 1979

Transposable elements are defined as specific DNA segments that can repeatedly insert into a few or many sites in a . They are classified as simple IS elements, more complex Tn transposons, and self-replicating episomes. Definitions and nomenclature rules for these three classes of prokaryotic transposable elements are specified.

I. INTRODUCTION modified in light of more recent develop- ments. The most important relevant tech- At the Cold Spring Harbor meeting on nical advances have been in the area of DNA insertions, a committee was elected nucleotide sequencing. In 1977, it seemed to draft a set of rules for nomenclature necessary that the primary operational (Campbell et al., 1977), These rules were definition for relatedness of two genetic based on discussions held at the meeting elements should be descent from a common and on previously accepted conventions for ancestor. Two elements should be identical the Mu (Howe and Bade, 1975), (or at least very similar) if descended in the for bacterial (Novick et al., 1976), laboratory from the same natural source. for A (Hershey, 1971), and for They are potentially different if derived bacterial (Demerec et al., 1966). from different sources. Common descent This article is a revised version of the becomes superhuous when complete nucleo- original report (Campbell et al., 1977) tide sequences of both elements are known (except insofar as the sequence of the ele- I In order to obtain wide distribution, this paper is ment used in a given experiment is gen- being published concurrently in , Vol. 5 (1979) No. 4. Suggestions for changes and amendments in erally inferred from that of some ancestor future versions of theserules should be directed to the on which the sequencing was performed). first author. Elements known to be identical in sequence

0147-619X/79/030466-08$02.00/0 466 Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved. NOMENCLATURE OF TRANSPOSABLE ELEMENTS 467 clearly should have the same name, even at random, to phage h, which usually, but when independently derived. not always, inserts at a single site in the Because of the range in size of known Escherichiu coli genome. Some (such as transposable elements (from 0.8 kb for IS2 A) have free phases that have been iden- to over 100 kb for some of the conjugative tified (e.g. intracellular replicons or extra- plasmids) we can anticipate an extended cellular ), whereas others (such as interval in which some of the shorter ele- the IS or Tn elements) are presently known ments (and perhaps numerous isolates only in the inserted state. thereof) will have been completely se- quenced, while sequence data on the larger III. CLASSES OF TRANSPOSABLE elements is still incomplete. We therefore ELEMENTS retain common descent as the only cri- terion for relatedness that is directly ap- ZZZ.A-l. IS Elements plicable to all transposable elements recog- nizing where appropriate the implications IS (Simple Insertion Sequences) elements of sequence information. contain no known unrelated to in- In the following report, the rules are pre- sertion function and are generally shorter sented in boldface type. Adherence to these than 2 kb. rules in all publications about inserting ele- ments is highly desirable. Interspersed with ZZZ.A-2. Symbols the rules are some comments, which include ISI, IQ, 1!33, I!34, IS5, etc. with the discussion and explanation of the rules, and also suggestions for possible usage in some numbers italicized. (Fiandt et al., 1972; Hirsch et al., 1972; MaIamy et al., 1972). cases not covered by the rules. The rules are intended to be minimal and applica- ZZZ.A-3. Comments ble to current or clearly imminent situations. We have not tried to anticipate problems At present, the IS elements listed above that may arise far in the future. We have are not known to contain any genes, either also avoided prescribing rules for problems related or unrelated to insertion function. that arise currently but rarely. Many of these Our intention in specifying “genes un- are best treated on an individual basis related to insertion function” is that even relevant to specific needs. if insertion genes should be discovered in some IS elements, they still would be II. TRANSPOSABLE ELEMENTS classified as simple insertion sequences. ZZ. A. Definition The term “gene” as used here means a DNA segment coding for a functional Transposable elements2 are specific DNA product and does not include, e.g., the “stop segmentsthat can repeatedly insert into a few signals” responsible for polar effects of IS or many sites in a genome. elements or promoters allowing IS elements to turn on adjacent genes. ZZ. B. Comments Operationally, the failure to detect other The definition applies to elements rang- genes in IS elements is closely tied to the ing from phage Mu, which seems to insert absence of a recognizable phenotype con- ferred by their presence. The only ob- served consequences of IS insertion are * The term “” is derived from cis-specific effects, such as changes in gene the work of McClintock (1952) on mobile genetic ele- ments in maize. In , an acceptable expression due to insertion within an operon, synonym is “translocation sequence” (Novick et or the rate of incidence of adjacent dele- al., 1976). tions. Effects other than c&specific ones 468 CAMPBELL ET AL.

are expected to be unobservable because complete base sequence of an IS insert has of the endogenous origin of IS elements. been determined, either it will prove They are always present in the , not only identical to some previously sequenced at the site under examination, but at other, insert or else minor variations in base generally unidentified sites as well. One or sequence may come to light. Here again it is more copy of each of the common IS ele- desirable to assign specific designations so ments is present in the E. coli genome. The that all inserts with identical sequences have phenotype of E. co/i lacking, for example, the same designation and are distinguished IS1 is unknown. from those known to differ from them. Because several copies of each element are present in the genome, it is currently ZZZ.B-l. Tn Elements (transposons) impossible to trace the lineage of a given IS element. When IS2 appears at a new site, Tn are more complex transposable ele- we cannot say which of the several pre- ments, often containing two IS elements. existing IS2s is ancestral to it. It must They behave formally like IS elements but therefore clearly be borne in mind that a are generally larger than 2 kb and contain name such as IS2 is a generic term referring additional genes unrelated to insertion to all insertions that appear indistinguish- function. able by hybridization, heteroduplexing, or restriction analysis, but which could have 111.B-2. Symbols individual differences in base sequence and Tnl , Tn2, Tnl, etc. with the numbers which might have undergone mutations or italicized. A different number is assigned to rearrangements in the laboratory (see IV. E) . each independent isolate from nature even if There is presently no advantage to giving it is apparently identical to some previous each IS2 in a different location a specific isolate by criteria other than direct de- as well as a generic name. Such a designa- termination of nucleotide sequence. If the tion would provide no additional informa- complete nucleotide sequenceof a new isolate tion not already given by the mutation is shown to be identical to that of some pre- number (see below) that identifies the in- vious isolate, the element should be designated sertion event. This situation may change in by the symbol assigned to the first isolate the future for two reasons: (1) IS2 elements with that sequence. If useful in a particular may be introduced into species from which context, genes carried by a Tn element may they are naturally absent. IS2, for example, be indicated following the name; e.g., Tn9 is said to be absent from Salmonella cam+ [Tn9 is a transposable element that typhimurium. If a single copy of IS2 is intro- confers chloramphenicol resistance (cam+)]. duced into S. typhimurium, then all sub- Mutations in Tn elements are designated by sequent transpositions within that species standard genetic symbols following the name must be derived from that specific IS2. If of the element, e.g., Tn!J cam4 is a particular and when such experiments are performed, mutant of Tn9 that no longer confers all IS2s derived from an identified IS2 in- chloramphenicol resistance. sertion should be assigned a specific desig- nation (e.g., IS2.1) to distinguish them from ZZZ.B-3. Comments IS2 insertions derived from another, po- tentially different source. (2) Laboratory Unlike IS elements, the lineage of Tn mutants of IS elements recognizably dif- elements generally can be followed unam- ferent from wild type have been isolated. biguously. Hence TnZ , Tn2, etc. are specific For example, Tomich and Friedman (1977) rather than generic terms. Every isolate reported a mutant of IS2 (IS2 rip2) that has labeled Tn2 either has a pedigree in the lost its polarity phenotype. (3) When the laboratory that traces back to the original NOMENCLATURE OF TRANSPOSABLE ELEMENTS 469

Tn2 isolate or has been shown to be identical Designations for some Tn elements de- to that isolate in base sequence. scribed in the published literature are given Note that the criterion is base sequence in Table 1. In Table 1, “plasmid origin” identity, not just extensive similarity. One indicates the natural plasmid from which might question whether this condition is per- the Tn element was derived. For example, haps too stringent, as it requires a new iso- Gottesman and Rosner (1975) studied trans- lation number for a transposon that differs positions of the chloramphenicol resistance from Tn2 even by one or two base pairs. determinant Tn9 from PlCm to A. Phage Such an element would clearly be related PlCm originated from growth of phage PI to Tn2 and might have arisen from Tn2 in a bacterium whose chloramphenicol re- by mutation in the laboratory. However, it sistance was derived from plasmid R14 is precisely in such cases that it is important (Kondo and Mitsuhashi, 1964). The R14 to keep straight which of the various Tn2- plasmid is now called pSM14 (Novick, 1974). like elements was used in a particular The Tn? element came either from pSC50, experiment. a derivative of RI (Kopecko and Cohen, In certain contexts, it may even be con- 1975) or has been transferred directly out venient to refer to a set of very similar of RI (Heffron et al., 1975a). transposons by some generic term. Since The designations assignedin Table 1 should the publication of our original report, some be used in all publications referring to ele- workers have in fact preferred to designate ments listed here. any or all of the ampicillin transposons Tnl - Tn2 as “TnA.” We prescribe no rules for such generic terms. They are not an appro- III. C. Episomes priate substitute for Tn numbers, as they provide no indication either of base sequence Episomes are complex, self-replicating ele- identity or of recent common ancestry. ments, often containing IS and Tn elements. Their use may be appropriate and con- Examples include bacteriophage A and the venient in the text of a scientific paper, pro- F plasmid of E. coli. No changes in exist- vided that the specific Tn number is in- ing designations for these elements are dicated when the element is first mentioned. proposed.

TABLE 1

Tn ELEMENTS

Transposable Plasmid Resistance element origin markers” Reference

Tnl RP4 AP Hedges and Jacob (1974) Tn2 RSF1030 AP Heffron et al. (1975b) Tn3 RI AP Kopecko and Cohen (1975) Tn4 Rl Ap,Sm,Su Kopecko and Cohen (1974) Tn5 JR67 Km Berg et al. (1975) Ttl6 JR72 Km Berg et al. (1975) Tn7 R483 TpSm Barth et al. (1975) Ttl9 pSM14 Cm Gottesman and Rosner (1975) TnlO RlOO Tc Foster et al. (1975); Kleckner et al. (1975) Tn551 ~1258 Em Pattee er al. (1977)

” Ap = amp = bla = ampicillin (p-lactamase); Sm = streptomycin; Su = sulfonamide; Km = kan = ka.na- mycin; Tp = trimethoprim; Tc = ret = ; Cm = cam = chloramphenicol; Em = erythromycin. 470 CAMPBELL ET AL

IV. AND WITH genetics, each published allele number must INSERTED ELEMENTS be unique. Individual laboratories engaged in parallel mutant isolations should en- IV. A. Insertion in a Particular Genome deavor by private communication to avoid When an inserting element has been intro- assignment of the same number to dif- duced into the genome of a previously de- ferent mutations prior to publication. (Num- scribed bacterial strain, or has been trans- bers 1 and 0 should generally be avoided posed within a strain, the new strain should because they often are confused with letters be given an isolation number. The genotype Land 0). of a strain that has acquired an inserting ele- ment can be denoted by the genotype of the IV. C. Comments parent strain, followed by the name of the element in parentheses: The rules are intentionally nonspecific as to designations for insertions of known ele- R126 = W3350(h) = FgafK2galTl(h). ments at unknown sites or unknown ele- Plasmids bearing IS or Tn elements should ments at known sites. For many purposes, be designated according to the rules de- strains carrying known elements at un- veloped for plasmid nomenclature (Novick et known sites are most conveniently treated al., 1976). In particular, each plasmid line as described above for insertions into a par- derived from an independent insertion event ticular genome, postponing a designation of should be assigned a new plasmid number. the specific insertion event until information on location is available. Some laboratories may find it convenient IV. B. Insertion at a Particular Site to serialize insertions using the Greek If location is known, specify gene or region capital omega to indicate that the number in which element is inserted, followed by a following refers to an insertion. This num- number designating the particular insertion ber would then be followed by a set of square mutation, then by a double colon, and brackets containing the available informa- finally by name of inserted element. tion about the nature and location of the insertion, thus: galT236::ISl Mutation 236 in the galT XYlOOO = C6OORlOO[K-12::Tn.5]Tn5 some- gene is an IS1 insertion. where in the K-12 genome, hisG34::TnZO Mutation 34 in the hisG including and plas- gene is a TnlO insertion. mids. galPO-E49O::W Mutation 490 between XYllOO = 8325fllOl[chr::Tn551] Tn551 galP0 and galE is an IS2 insertion. somewhere in the host chromo- AP-Qbl::IS2 Mutation b 1 between genes some. P and Q is an IS2 insertion. XY1200 = C6000102[galT236::IS1] IS1 F8 42kb-7::Tn2 Mutation 7 at 42 kb on F8 within chromosomal gene galT. plasmid is a Tn2 insertion. XY1300 = C600[pSC101s1103[4.5kb::TnZ]] Designation of new insertion mutations Tnl at 4.5 kb on the pSClO1 should be by number. Previously published plasmid. symbols may be used for mutations already Note that the assignment of both a strain named in the literature, e.g., MS348, r32, number and an insertion number may in bi2, erg , etc. Old symbols should be changed some cases be redundant; however, if the only when it is generally agreed that they DNA segment carrying the insertion is trans- violate the rules “excessively” (Demerec ferred to a new strain, the insertion num- et al., 1966). As in other areas of prokaryotic ber should be retained since there has not NOMENCLATURE OF TRANSPOSABLE ELEMENTS 471 been a new insertion, but the resulting strain For 1%) both orientations are polar. IS2 would be a new one. Note also that as more in orientation II is frequently constitutive information about the insertion becomes (antipolar) because of readthrough from an available, this can simply be included in- IS2 promoter (Fiandt et al., 1972; Saedler side the square brackets; retention of the et al., 1974). insertion number will provide assurance of the identity of the genetic segment in IV. E. Mutations in Inserted Elements question. Mutations are listed after the symbol for Insertion of an uncharacterized element the element: at a known site should be indicated simply by a gene designation and mutant number. hisG1327::TnlO tet43, The insertional nature of the mutation should y r32::IS2 rip2. not be part of its name unless or until the insertion has been identified; e.g., biop,l3 1 IV. F. Complex Genome Designations (insertion of unknown DNA into the biotin promoter). When the insertion and its loca- Genetic symbols for a strain having two in- tion are better characterized, the designa- sertions and other mutations should be written tion should be modified, accordingly (e.g., in the same order as mapped on the genome: biop,l31::ISI). hWam43 62 xis int-c60::IS2 ~1857 IV. D. Orientation PQbl::IS2 Sam7. Where the orientation of the insertion is If polarity is indicated, the above strain known, it may be desired to include that would be orientation in the strain description. Orienta- tion may be specific either (a) with respect hWam43 b2 xis int-c60::IS2. + ~1857 to direction along the genome or (b) with P-Qb 1: :IS2 * + Sam7 (genome polarity) respect to the polarity of the operon. Following (with a slight modification) or the conventions adopted for phage Mu, hWam43 62 xis int-c60::IS2 *II ~1857 orientation with respect to the genome is in- dicated by a plus (+) or minus (-) sign fol- P-Qbl::IS2 .I Sam7 (operon polarity). lowing the symbol of the element. For ex- When both the inserted element and the ample, proA : : Mu. + . (A plus orientation genome into which it is inserted carry muta- means that if one moves around the E. co/i tions, the symbols for the inserted element and map in a clockwise motion, the immunity its mutations may be set off by parentheses: end of the Mu prophage will be reached first and the S end last [Howe and Bade, hWam43 y r32::(IS2 rip2)*1 Sam7. 19751). For IS and Tn elements, the plus V. CENTRAL REGISTRY orientation is arbitrary and the minus orientation is opposite to it. To avoid duplication of numbers, all new Orientation with respect to the operon may IS and Tn elements should be checked with a be indicated with roman numerals I and II. central registry before numerals are as- Orientation I is assigned arbitrarily, usually signed to them in publications. when a new IS element is first characterized A registry for Tn elements is maintained in an operon with known polarity: by Dr. , Department of Medical Microbiology, Stanford University lacZ348::ISZ ‘I, Medical School, Stanford, California 94305, xis int-c60::IS2 .II. as part of the Plasmid Reference Center. 472 CAMPBELL ET AL.

TABLE 2 REGISTRYOF Tn NUMBER ALLOCATIONSTHROUGH FEBRUARY 1979

Tn Nos. Laboratory Location

11-20 D. Botstein Cambridge, Mass., U. S. A. 21-70 S. N. Cohen Stanford, Calif., U. S. A. 71-100 N. Datta London, England 101-200 D. Botstein Cambridge, Mass., U. S. A. 201-300 S. N. Cohen Stanford, Calif., U. S. A. 301-400 L. Rosner Bethesda, Md., U. S. A. 401-500 J. Shapiro Chicago, Ill., U. S. A. 501-520 V. Stanisich Melbourne, Australia 521-550 D. Taylor, R. Grant Toronto, Ontario, Canada 551-600 R. P. Novick New York, N. Y., U. S. A. 601-700 J. Davies, D. E. Berg Madison, Wise. and St. Louis, MO., U. S. A. 701-750 N. Datta London, England 751-800 Hammersmith Hospital London, England 801-850 P. Bennett Bristol, England 851-900 M. Syvanen Cambridge, Mass., U. S. A. 901 E. Veltkamp Amsterdam, Holland 902 J. Scott Atlanta, Ga., U. S. A. 903 A. Oka Kyoto, Japan 904-915 R. Olsen Ann Arbor, Mich., U. S. A. 926-950 D. Kopecko, J. Wohlheiter Washington, D. C., U. S. A. 951-1000 H. Saedler Freiburg, West Germany lOOl- 1099 M. Guyer Bethesda, Md., U. S. A. 1loo- 1400 S. Falkow Seattle, Wash., U. S. A. 1401- 1500 A. Summers, G. Jacoby Boston, Mass., U. S. A. 1501-1520 A. I. Bukhari Cold Spring Harbor, N. Y., U. S. A. 1521-1600 P. Courvalin Paris, France 1601-1640 P. Kontomichalou Athens, Greece l&11- 1660 M. Van Montagu, J. Schell Ghent , Belgium 1681-1695 F. Heffron San Francisco, Calif., U. S. A. 1696- 1700 C. Rubens Seattle, Wash., U. S. A. 1701-1716 D. Nakada Pittsburgh, Pa., U. S. A. 1721-1750 R. Schmitt Regensburg, West Germany 1751-1770 G. Smirnov Moscow, U. S. S. R. 1771-1800 F. Schoffl Erlangen, West Germany 1801- 1820 0. Marquardt Hamburg, West Germany 1821-1830 H. Tschape Wernigerode (Harz), East Germany 1831-1840 G. Lebek Berne, Switzerland 1841-1.860 B. Davey Melbourne, Australia -

Blocks of numbers have been assigned to REFERENCES investigators on request. Some of these assignments are listed in Table 2. BARTH, P. T., DATTA, N., HEDGES, R. W., AND GRINTER, N. J. (1976). Transposition of a deoxy- ribonucleic acid sequence encoding trimethoprim ACKNOWLEDGMENTS and streptomycin resistances from R483 to other replicons. J. Bacten’ol. 125, 800-810. The authors are grateful to Stanley Cohen and to many other individuals for suggestions and informa- BERG, D. E., DAVIES, J., ALLET, B., AND ROCHAIX,J. tion. The activities of the Plasmid Reference Center (1975). Transposition of R factor genes to bacterio- are supported by NIH Grant AI-13093 and Contract phage A. Proc. Nat. Acad. Sri. USA 72, 3628- Not-AI-72531. 3632. NOMENCLATURE OF TRANSPOSABLE ELEMENTS 473

CAMPBELL, A., BERG, D., BOTSTEIN, D., LEDER- KLECKNER,N., CHAN, R., TYE, B., AND BOTSTEIN,D. BERG, E., NOVICK, R., STARLINGER, P., AND (1975). Mutagenesis by insertion of a drug-re- SZYBALSKI, W. (1977). Nomenclature of trans- sistance element carrying an inverted repetition. posable elements in prokaryotes. In DNA lnser- J. Mol. Biol. 97, 561-575. tion Elements, Plasmids and Episomes, (A. I. KONDO, E., AND MITSUHASHI, S. (1964). Drug Bukhari, J. S. Shapiro, and S. L. Adhya, S. L. eds.), resistance of enteric . IV. Active transduc- pp. 15-22. Cold Spring Harbor Laboratory, Cold ing bacteriophage PlCM produced by the combina- Spring Harbor, N. Y. tion of R factor with bacteriophage PI. J. Bacterial. DEMEREC,M., ADELBERG, E. A., CLARK, A. J., AND 88, 1266- 1276. HARTMAN, P. E. (1966). A proposal for a uniform KOPECKO, D. J., AND COHEN, S. N. (1975). Site- nomenclature in bacterial genetics. Generics 54, specific recA-independent recombination between 61-76. bacterial plasmids: Involvement of palindromes at FIANDT, M., SZYBALSKI, W., AND MALAMY, M. H. the recombinational loci. Proc. Nat. Acad. Sci. (1972). Polar mutations in lac, gal, and phage A USA 72, 1373-1377. consist of a few IS DNA sequences inserted with MALAMY, M. H., FIANDT, M., AND SZYBALSKI, W. either orientation. Mol. Gen. Gene?. 119, 223-231. (1972). Electron microscopy of polar insertions in FOSTER, T. J., HOWE, T. G. B., AND RICHMOND, the lac operon of . Mol. Gen. M. H. (1975). Translocation of the tetracycline Genet. 119, 207-222. resistance determinant from RIOO-1to the Escherichia MCCLINTOCK, B. (1952). Chromosome organization coli K-12chromosome.J. Bacterial. 124,1153- 1158. and . Cold Spring Harbor Symp. GOTTESMAN,M., AND ROSNER, J. L. (1975). Acquisi- Quanr. Biol. 16, 13-47. tion of a determinant for chloramphenicol re- NOVICK, R. P. (1974). Bacterial plasmids. In “Hand- sistance by coliphage h. Proc. Nat. Acad. Sci. book of Microbiology” (A. I. Laskin and H. A. USA 72,5041-5045. Lechevalier, eds.), Vol. IV, pp. 537-586. Chem. HEDGES, R. W., AND JACOB,A. E. (1974). Transposi- Rubber Co., Cleveland. tion of ampicillin resistance from RP4 to other NOVICK, R. P., CLOWES, R. C., COHEN, S. N., replicons. Mol. Gen. Genet. 132, 31-40. CURTISS, R., Ill, DATTA, N., AND FALKOW, S. HEFFRON, F., RUBENS,C., AND FALKOW, S. (1975a). (1976). Uniform nomenclature for bacterial plas- Translocation of a plasmid DNA sequence which mids: A proposal. Bacterial. Rev. 40, 168-189. mediates ampicillin resistance: Molecular nature and PATTEE, P. A., THOMPSON,N. E., HAUBRICH, D., AND specificity of insertion. Proc. Nat. Acad. Sci. USA NOVICK, R. P. (1977). Chromosomal map locations 72, 3623-3627. of integrated plasmids and related elements in HEFFRON, F., SUBLETT, R., HEDGES, R. W., JACOB, . Plasmid 1, 38-51. A., AND FALKOW, S. (1975b). Origin of the TEM SAEDLER, H., REIF, H. J., Hu, S., AND DAVIDSON, N. beta lactamase gene found on plasmids. J. Bacreriol. (1974). lS2, a genetic element for turn-off and 122, 250-256. turn-on of gene activity in E. coli. MO/. Gen. HERSHEY, A. D. (Ed.) (1971). “The Bacteriophage Genet. 132, 265-289. Lambda.” Cold Spring Harbor Laboratory, Cold TOMICH, P. K., AND FRIEDMAN, D. I. (1977). lsola- Spring Harbor, N. Y. tion of mutations in insertion sequences that re- HIRSCH, H.-J., STARLINGER, P., AND BRACHET, P. lieve IS-induced polarity. In “DNA Insertion Ele- (1972). Two kinds of insertions in bacterial genes. ments, Plasmids, and Episomes” (A. I. Bukhari, Mol. Gen. Genet. 119, 191-206. J. A. Shapiro, and S. L. Adhya, eds.), pp. 99-107. HOWE, M., AND BADE, E. (1975). Cold Spring Harbor Laboratory, Cold Spring of . Science 190, 624-632. Harbor, N. Y.