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In Vivo Interactions of Archaeal Cdc6 Orc1 and Minichromosome

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In Vivo Interactions of Archaeal Cdc6 Orc1 and Minichromosome In vivo interactions of archaeal Cdc6͞Orc1 and minichromosome maintenance proteins with the replication origin Fujihiko Matsunaga*, Patrick Forterre*, Yoshizumi Ishino†, and Hannu Myllykallio*§ *Institut de Ge´ne´ tique et Microbiologie, Universite´de Paris-Sud, 91405 Orsay, France; and †Department of Molecular Biology, Biomolecular Engineering Research Institute, 6-2-3 Furuedai, Suita, Osaka 565-0874, Japan Communicated by Bruce W. Stillman, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, July 25, 2001 (received for review May 25, 2001) Although genome analyses have suggested parallels between basis of asymmetry in base composition between leading and archaeal and eukaryotic replication systems, little is known about lagging strands (10, 11). In Pyrococcus abyssi, the location of the the DNA replication mechanism in Archaea. By two-dimensional predicted origin coincides with an early replicating chromosomal gel electrophoreses we positioned a replication origin (oriC) within segment of 80 kb identified by radioactive labeling of chromo- 1 kb in the chromosomal DNA of Pyrococcus abyssi, an anaerobic somal DNA in cultures released from a replication block (12). hyperthermophile, and demonstrated that the oriC is physically Our in silico and labeling data also allowed us to conclude that linked to the cdc6 gene. Our chromatin immunoprecipitation as- the hyperthermophilic archaeon P. abyssi uses a single bidirec- says indicated that P. abyssi Cdc6 and minichromosome mainte- tional origin to replicate its genome. We proposed that this origin nance (MCM) proteins bind preferentially to the oriC region in the would correspond to the long intergenic region conserved in all exponentially growing cells. Whereas the oriC association of MCM three known Pyrococcus sp. genomes directly 5Ј upstream of was specifically inhibited by stopping DNA replication with puro- cdc6, which contains sequence elements similar to those found mycin treatment, Cdc6 protein stayed bound to the replication in the other chromosomal replication origins. Our data also origin after de novo protein synthesis was inhibited. Our data indicated that the replication speed in Archaea is similar to that suggest that archaeal and eukaryotic Cdc6 and MCM proteins in Bacteria and up to 10 times higher than in Eukarya. Therefore, function similarly in replication initiation and imply that an oriC despite their very different replication proteins, in terms of association of MCM could be regulated by an unknown mechanism replication speed and origin utilization, the two prokaryotic in Archaea. domains (i.e., Bacteria and Archaea) share a similar replication mode, whereas Eukarya use multiple replication origins to Archaea ͉ minichromosome maintenance helicase ͉ origin accurately replicate their much larger linear chromosomes. How association ͉ initiator protein subsets of archaeal replication proteins, either similar to eukary- otic proteins or limited to Archaea, like, e.g., a novel type II he initiation step of chromosomal DNA replication is of DNA topoisomerase (13) and a family D DNA polymerase (14), Tfundamental importance for the inheritance of genetic ma- are used for replication in Archaea remains unclear. Thus, in terial and cell cycle regulation and therefore has attracted addition to the bacterial features described above, the archaeal ͞ considerable experimental interest over the years. Detailed replication mechanism could consist of eukaryotic and or yet to studies of DNA replication initiation in Eukarya and Bacteria be discovered unique archaeal features. have identified multistep processes leading to the unwinding of Pyrococcus species are unique among those Archaea for which duplex DNA at a replication origin (recently reviewed in refs. 1 full-length genome sequences are available, in that they contain and 2). In comparison with the relatively detailed knowledge only a single gene similar to eukaryotic Cdc6 and the Orc1 ͞ obtained for Bacteria and Eukarya with regard to their repli- subunit of the origin recognition complex (ORC) (Cdc6 Orc1, cation initiation, until recently, no data were available on the referred to hereafter as Cdc6 for simplicity) as well as the MCM replication mechanism in Archaea, the third domain of life. proteins. These proteins are the only known candidate initiator ͞ Because many eukaryotic replication proteins, including the proteins present in these species. With the use of neutral neutral ͞ initiator proteins Orc1, Cdc6, and minichromosome mainte- two-dimensional (N N 2D) agarose gel electrophoreses, the Ϸ nance (MCM), have homologs in Archaea but not in Bacteria defined origin was found in a 10-kb window in the vicinity of (3–6), studies of the archaeal replication system have the po- genes encoding several replication proteins [i.e., the Cdc6, two tential to elucidate hitherto unresolved questions with regard to subunits of the family D DNA polymerase, and two subunits of the eukaryotic replication mechanism. In the absence of bio- a clamp loader (RF-C)], in accordance with our earlier proposal chemical or genetic in vivo data, whether the archaeal initiator for the origin location in P. abyssi (12). With the use of in vivo proteins are functionally analogous to their eukaryotic counter- chemical cross-linking assays we also have investigated the parts has remained elusive. Nevertheless, the recent demonstra- specific origin association of P. abyssi Cdc6 and MCM. These tion that the single MCM protein of the archaeon Methanobac- studies revealed that P. abyssi Cdc6 and MCM proteins associate terium thermoautotrophicum forms a double-homohexamer specifically with origin DNA in exponential phase cultures. complex with a processive 3Ј to 5Ј ATP-dependent DNA helicase Whereas origin association of P. abyssi Cdc6 was maintained, activity in vitro (7–9) has illustrated how identifying the prop- association of MCM was abolished in cells not replicating their erties of an archaeal replication protein can aid in understanding DNA. These data are experimental proof that, as in Eukarya, processes leading to unwinding of the replication origin by eukaryotic replication proteins. Abbreviations: MCM, minichromosome maintenance; ORC, origin recognition complex; For a long time, the study of archaeal DNA replication in vivo N͞N 2D, neutral͞neutral two-dimensional. and in vitro has been hampered by the absence of information §To whom reprint requests should be addressed. E-mail: hannu.myllykallio@igmors. about the nature of the replication origins in these atypical u-psud.fr. prokaryotes. Complete archaeal genome sequences have made The publication costs of this article were defrayed in part by page charge payment. This it possible to predict the location of a single replication origin in article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. M. thermoautotrophicum and Pyrococcus sp. genomes on the §1734 solely to indicate this fact. 11152–11157 ͉ PNAS ͉ September 25, 2001 ͉ vol. 98 ͉ no. 20 www.pnas.org͞cgi͞doi͞10.1073͞pnas.191387498 Downloaded by guest on October 2, 2021 Chromatin Binding and Detection of Association Between the P. abyssi oriC and Archaeal Initiator Proteins. Purification of P. furiosus Cdc6 and MCM proteins after their production in Escherichia coli and polyclonal antibodies raised against these proteins has yet to be reported (Y.I., S. Ishino, M. Nakae, and K. Komori, unpublished data). Antisera thus obtained recognized one major chromatin- associated band of approximately expected molecular weight in cell-free extracts of P. furiosus and were shown to cross-react with the corresponding P. abyssi proteins. For chromatin asso- ciation studies, P. abyssi GE5 cells were harvested in the late exponential phase, 5–6 h after inoculation. Stationary phase cultures were collected after overnight cultivation. Samples were Fig. 1. Physical map of a P. abyssi chromosomal segment (sequence coordi- ͞ nates 118,000–128,000) carrying the predicted replication origin (12). Loca- washed once with washing buffer (25 mM Hepes, pH 7.0 1M Ϸ 9 tions of several restriction sites (H, HindIII; N, NheI; E, EcoRI; X, XbaI), probes sorbitol). Cell suspensions ( 10 cells per milliliter) were dis- used for N͞N 2D analyses are indicated. Fragments A (4.9-kb NheI fragment), rupted in extraction buffer (25 mM Hepes, pH 7.0͞15 mM ͞ ͞ ͞ ͞ B (5.7-kb EcoRI fragment), C (4.1-kb HindIII fragment), and D (5.6-kb XbaI MgCl2 100 mM NaCl 0.4 M sorbitol 0.5% Triton X-100 1 fragment) correspond to those analyzed in Fig. 2. The central third of the each mM PMSF͞0.5 ␮g/ml leupeptin͞1 ␮g/ml pepstatin A). After 10 fragment [a bubble detection zone (18)] is indicated by gray shading. A Ϸ1-kb min on ice, soluble proteins (supernatant) and chromosomal boxed region upstream of, and partially overlapping with, the P. abyssi cdc6 DNA-enriched insoluble fraction (pellet) were separated by indicates the region containing an active replication origin as demonstrated centrifugation at 14,000 ϫ g for 20 min. The insoluble fraction, in Fig. 2. containing DNA and chromatin-associated proteins, was washed once and dissolved in an equal volume of extraction buffer by brief sonication (45 W, four times for 15 s each), yielding DNA Cdc6 and MCM helicase form a core complex required for fragments of 500–1,000 bp. The obtained fractions were ana- replication initiation in Archaea. lyzed by immunoblotting with polyclonal rabbit antiserums. An Materials and Methods enhanced chemiluminescence kit (Amersham Pharmacia) was used to detect immunocomplexes
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