82 NATURE VOL. 231 MAY 14 1971 SUPPRESSION the vermilion and suppressor loci are he bas shown that the addition of un­ both on the X chromosome, only the charged tRNATl' to the tryptophan Drosophila suppressor locus controls the loss of the pyrrolase of vermilion , from our Cell Biology Correspondent tyrosine tRNA. The recessive nature activated by ribonuclease digestion, BIOLOGISTS who enjoy vicarious plea­ of the suppressor mutations and the once more inhibits the enzyme's activity. sures should not miss reading the cur­ fact that tRNA structural are re­ This species of tRNA is therefore an rent issue of the Journal of Molecular dundant in eukaryotes indicate that the inhibitor of the vermilion Biology (57, 231 ; 1971), for in it suppressor locus is not the structural enzyme but it does not inhibit wild type Twardzik, Grell and Jacobson report for the tRNA in question but the enzyme. suppressor locus presumably specifies an that a suppressor , which sup­ From all this Jacobson argues that presses the vermilion eye colour muta­ enzyme which somehow modifies this the wild type enzyme is associated in tRNA during its maturation. tion of Drosophila melanogaster, alters some way with tRNA ~ and is active But how does the loss of the second a species of the fly's tyrosine tRNAs. whereas the tryptophan pyrrolase speci­ Mention of genetic suppression to tyrosine tRNA, tRNA V', suppress the fied by the vermilion locus, presumably vermilion mutation? At the time anybody reared on the biology of because of its mutated structure, is in­ Escherichia coli conjours up, of course, Twardzik and his colleagues wrote their hibited when it associates with this report they were clearly thinking along pictures of mutated transfer RNA mole­ tRNA. Removal of the tRNA either conventional lines, speculating about cules which, either because they have a by nuclease digestion or by introducing possible ambiguous codon responses of new anticodon, or as a result of some the suppressor su(sf mutation, which the unmodified tRNA'T, and suppres­ other structural modification, have prevents the maturation of tRNA'T, sion at the level of . But gained the ability to read nonsense chain leads to the reactivation of the mutant terminating codons or to rectify mis­ further experiments reported recently by enzyme. Clearly this is the first of a Jacobson in Nature New Biology (Z31, sense mutations. Fully aware that sup­ fascinating collection of stories. Why pression in microorganisms is usually 17; 1971) have changed all that. so many isoaccepting species of tRNAs mediated by a structurally changed Jacobson has found that by treating exist has long been a puzzle but there tRNA, Twardzik and his colleagues homogenates of vermilion mutant flies is now a clue as to their function. They simply asked if the same might not be with ribonuclease T1 be was able to re­ may well be more important for the store their tryptophan pyrrolase activity. true of suppression in higher organisms, control of enzyme activities-in other The obvious implication was that an about the mechanism of which nothing words the control of metabolism and is known; their hunch has paid off RNA inhibits tryptophan pyrrolase in differentiation-than the control of handsomely, albeit in an unexpected vermilion mutants. Pursuing this lead translation. way. Geneticists long since isolated a re­ cessive sex-linked suppressor mutation, su(sf, which is non-allelic to the sex­ linked vermilion eye colour mutation of Intricacies of Slaning a Drosophila, which it suppresses. Dro­ IN Nature New Biology next Wednes­ Rudland and his colleagues a different sophila carrying the vermilion mutation day, Rudland, Whybrow and Clark factor, the F2 initiation factor, is in­ lack a brown eye pigment because they suggest that a protein called initiation volved in the delivery of the very first are unable to convert tryptophan to factor F2 plays a crucial part in charged tRNA which initiates protein kynurenine, an intermediate step, cata­ ensuring that the transfer RNA carry­ synthesis. They have shown that F2 lysed by tryptophan pyrrolase, in the ing the amino-acid which initiates the protein will form a complex with a synthesis of the brown pigment. In synthesis of all Escherichia coli molecule of GTP and a molecule of these mutant flies tryptophan pyrrolase goes to the correct site in a ribosome. the initiator transfer RNA charged activity is never more than 25 per cent During protein synthesis each trans­ with the initiating amino-acid, formyl­ that in wild type flies but when the fer RNA molecule picks up a par­ methionyl-tRNA1. They envisage that su(sf suppressor mutation is introduced ticular amino-acid, carries it to the F2, in this complex, plays a part analo­ the activity of this enzyme is partially ribosome and, if the codon for that gous to T factor, but instead of deliver­ restored, the large accumulation of non­ amino-acid is waiting to be read, ing the formylmetbionyl-tRNA1 to the protein tryptophan, which characterizes delivers it to the site at which amino­ A-site of a ribosome it delivers the the vermilion mutants, is reduced, and acids are added to a growing polypep­ initiator to the P-site. eye colour is restored. tide chain. Clearly, ensuring that the Once the formylmethionyl-tRNA1 Taking the bull by the horns, charged tRNA is delivered to precisely is bound to the P-site the second and Twardzik et al. screened extracts of flies, the right site in the ribosome is of subsequent amino-acids attached to searching by reverse phase chromato­ crucial importance and it is known their tRNAs can be delivered by the T graphy for a modified species of tRNA that a protein called T factor promotes factor to the A-site and added to the in the su(s)2 homozygotes; and sure this reaction. growing chain. And to ensure that enough they found that the su(s)2 locus T factor will bind with a mole­ only the initiator species of transfer controls directly the amount of a species cule of charged tRNA and a molecule RNA enters the P-site the two factors of tyrosine tRNA in adult flies. Wild of GTP to form a complex; this com­ T and F2 have evolved mutually type Drosophila have three resolvable plex then delivers the tRNA complete exclusive specificities. T factor will tyrosine tRNAs, two major species and with its specific amino-acid to what is complex with any charged tRNA except a minor, and su(s)2 flies lack the second called the A-site of a ribosome. The the initiator species, while factor F2 major species and have a larger than A-site is the reception centre for will only complex with, and therefore normal amount of tRNA chromato­ charged tRNA molecules which can deliver, the initiator formylmethionyl­ graphing at the position of the first donate the amino-acid they carry to tRNA1. Having hit on a successful major species. Flies bearing a new the growing protein held in the so­ basic mechanism for the delivery of suppressor mutation su(sje 1 isolated by called P-site. charged tRNAs to ribosomes, nature Twardzik et al., which maps at the same But T factor only promotes the bas evolved two neat variations to make locus as su(sf, also lack the second delivery of charged tRNAs to a ribo­ certain that the processes of starting a major tyrosine tRN A. A series of some which has already started to protein and elongating a protein do not genetic crosses established that although synthesize a protein. According to interfere with each other.

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