The metC gene of Escherichia coli K-12 was cloned and the nucleotide sequence of the metC gene and its flanking regions was determined. The translation initiation codon was identified by sequencing the NH2-terminal part of 13-cystathionase, the MetC gene product. The meIC gene (1185 nucleotides) encodes a protein having 395 amino acid residues.The 5' noncoding region was found to contain a "Met box" homologous to sequences suggestive of operator structures upstream from other methionine genes that are controlled by the product of the pleiotropic regulatory metJ gene. The deduced amino acid sequence of (3-cystathionase showed extensive homology with that of the MetB protein (cystathionine y-synthase) that catalyzes the preceding step in methionine biosynthesis. The homology strongly suggests that the structural genes for the MetB and MetC proteins evolved from a common ancestral gene.

Reverse gyrases are atypical topoisomerases present in hyperthermophiles and are able to positively supercoil a circular DNA. Despite a number of studies, the mechanism by which they perform this peculiar activity is still unclear. Sequence data suggested that reverse gyrases are composed of two putative domains, a helicase-like and a topoisomerase I, usually in a single polypeptide. Based on these predictions, we have separately expressed the putative domains and the fulllength polypeptide of Sulfolobus acidocaldarius reverse gyrase as recombinant proteins in Escherichia coli. We show the following.

Deinococcus radiodurans is known for its extreme radioresistance. Comparative genomics identified a radiation-desiccation response (RDR) regulon comprising genes that are highly induced after DNA damage and containing a conserved motif (RDRM) upstream of their coding region. We demonstrated that the RDRM sequence is involved in cis-regulation of the RDR gene ddrB in vivo. Using a transposon mutagenesis approach, we showed that, in addition to ddrO encoding a predicted RDR repressor and irrE encoding a positive regulator recently shown to cleave DdrO in Deinococcus deserti, two genes encoding α-keto-glutarate dehydrogenase subunits are involved in ddrB regulation. In wild-type cells, the DdrO cell concentration decreased transiently in an IrrE-dependent manner at early times after irradiation. Using a conditional gene inactivation system, we showed that DdrO depletion enhanced expression of three RDR proteins, consistent with the hypothesis that DdrO acts as a repressor of the RDR regulon. DdrO-depleted cells loose viability and showed morphological changes evocative of an apoptotic-like response, including membrane blebbing, defects in cell division and DNA fragmentation. We propose that DNA repair and apoptotic-like death might be two responses mediated by the same regulators, IrrE and DdrO, but differently activated depending on the persistence of IrrE-dependent DdrO cleavage.

No abstract

The nucleoid of radioresistant bacteria, including D. radiodurans, adopts a highly condensed structure that remains unaltered after exposure to high doses of irradiation. This structure may contribute to radioresistance by preventing the dispersion of DNA fragments generated by irradiation. In this report, we focused our study on the role of HU protein, a nucleoid-associated protein referred to as a histone-like protein, in the nucleoid compaction of D. radiodurans. We demonstrate, using a new system allowing conditional gene expression, that HU is essential for viability in D. radiodurans. Using a tagged HU protein and immunofluorescence microscopy, we show that HU protein localizes all over the nucleoid and that when HU is expressed from a thermosensitive plasmid, its progressive depletion at the non-permissive temperature generates decondensation of DNA before fractionation of the nucleoid into several entities and subsequent cell lysis. We also tested the effect of the absence of Dps, a protein also involved in nucleoid structure. In contrast to the drastic effect of HU depletion, no change in nucleoid morphology and cell viability was observed in dps mutants compared with the wild-type, reinforcing the major role of HU in nucleoid organization and DNA compaction in D. radiodurans.

The Deinococcus radiodurans bacterium exhibits an extreme resistance to ionizing radiation. Here, we investigated the in vivo role of DdrB, a radiation-induced Deinococcus specific protein that was previously shown to exhibit some in vitro properties akin to those of SSB protein from E. coli but also to promote annealing of single stranded DNA. First we report that the deletion of the C-terminal motif of the DdrB protein, which is similar to the SSB C-terminal motif involved in recruitment to DNA of repair proteins, did neither affect cell radioresistance nor DNA binding properties of purified DdrB protein. We show that, in spite of their different quaternary structure, DdrB and SSB occlude the same amount of ssDNA in vitro. We also showed that DdrB is recruited early and transiently after irradiation into the nucleoid to form discrete foci. Absence of DdrB increased the lag phase of the extended synthesis-dependent strand annealing (ESDSA) process, affecting neither the rate of DNA synthesis nor the efficiency of fragment reassembly, as indicated by monitoring DNA synthesis and genome reconstitution in cells exposed to a sub-lethal ionizing radiation dose. Moreover, cells devoid of DdrB were affected in the establishment of plasmid DNA during natural transformation, a process that requires pairing of internalized plasmid single stranded DNA fragments, whereas they were proficient in transformation by a chromosomal DNA marker that integrates into the host chromosome through homologous recombination. Our data are consistent with a model in which DdrB participates in an early step of DNA double strand break repair in cells exposed to very high radiation doses. DdrB might facilitate the accurate assembly of the myriad of small fragments generated by extreme radiation exposure through a single strand annealing (SSA) process to generate suitable substrates for subsequent ESDSA-promoted genome reconstitution.

Investigation of the presence of a reverse gyrase-like activity in archaebacteria revealed wide distribution of this activity in hyperthermophilic species, including methanogens and sulfur-dependent organisms. In contrast, no reverse gyrase activity was detected in mesophilic and moderately thermophilic organisms, which exhibited only an ATP-independent activity of DNA relaxation. These results suggest that the presence of reverse gyrase in archaebacteria is tightly linked to the high growth temperatures of these organisms. With respect to antigenic properties, the enzyme appeared similar among members of the genus Sulfolobus. In contrast, no close antigenic relatedness was found between the reverse gyrase of members of the order Sulfolobales and that of the other hyperthermophilic organisms.On the basis of partial rRNA sequence comparisons, archaebacteria have been divided into two major branches: the sulfur-dependent and the methanogenic plus halophilic archaebacteria, including the genus Thermoplasma (37, 38). Thermophilic archaebacteria are found in these two branches; indeed, although most of them are clustered in the first group (32), thermophilic and hyperthermophilic species are also found in the group of methanogens. Likewise, Archaeoglobus, a genus that occupies an intermediate position between the two major groups, is also an extremely thermophilic archaebacterium, with an optimum growth temperature at 830C (1,29,31,39).Biochemical mechanisms allowing growth at very high temperatures are still not well known. A few years ago, a singular topoisomerase, the reverse gyrase, capable of introducing positive supercoils into closed circular plasmid DNA was isolated from two sulfur-dependent hyperthermophilic archaebacteria, Sulfolobus acidocaldarius (9, 17) and Desulfurococcus amylolyticus (28). The enzyme purified from these two organisms is made of a single polypeptide of about 120 to 135 kDa (21,23,28). It is the only known topoisomerase that exhibits an activity of reverse gyration (positive supercoiling of DNA) per se and that is both a type I and an ATP-dependent topoisomerase (9, 23). Furthermore, the DNA of the viruslike particle SSV1 (present in Sulfolobus sp. strain B12) was found to be positively supercoiled after extraction (22), suggesting that the reverse gyrase activity takes place in vivo. From these results, it seemed interesting to determine whether positive supercoiling of DNA was essential for life at high temperatures and whether the presence of reverse gyrase reflected a property of a phylogenetically restricted group of organisms. In the latter case, analysis of the distribution of reverse gyrase within the archaebacterial kingdom could constitute a tool to confirm the phylogenetic position of some archaebacteria.We have examined the presence of reverse gyrase in a large variety of strains, including sulfidogens and methanogens. We found the reverse gyrase activity (i.e., ATP- (about 2 x 10-3 U), the mixture was incubated for 15 min at 75°C. The reaction was stopped by quick cooling an...

The hyperthermophilic bacterium Thermotoga maritima MSB8 possesses a reverse gyrase whose enzymatic properties are very similar to those of archaeal reverse gyrases. It catalyzes the positive supercoiling of the DNA in an Mg 2؉ -and ATP-dependent process. Its optimal temperature of activity is around 90°C, and it is highly thermostable. We have cloned and DNA sequenced the corresponding gene (T. maritima topR). This is the first report describing the analysis of a gene encoding a reverse gyrase in bacteria. The T. maritima topR gene codes for a protein of 1,104 amino acids with a deduced molecular weight of 128,259, a value in agreement with that estimated from the denaturing gel electrophoresis of the purified enzyme. Like its archaeal homologs, the T. maritima reverse gyrase exhibits helicase and topoisomerase domains, and its sequence matches very well the consensus sequence for six reverse gyrases now available. Phylogenetic analysis shows that all reverse gyrases, including the T. maritima enzyme, form a very homogeneous group, distinct from the type I 5 topoisomerases of the TopA subfamily, for which we have previously isolated a representative gene in T. What are the molecular mechanisms involved in the adaptation of life to elevated temperatures? In terms of DNA dynamics, part of the answer was provided by the discovery in thermophilic organisms of a particular topoisomerase, the reverse gyrase, that modifies the topological state of DNA by introducing positive supercoils in an ATP-dependent process (14). It was suggested that overlinking could compensate for the effect of temperature on DNA structure (16). The enzyme is widely distributed in thermophilic archaea (6,8). The first reverse gyrase characterized was isolated from the hyperthermophilic archaeum Sulfolobus acidocaldarius (23,33). Mechanistic studies showed that it is transiently linked to the DNA by a 5Ј phosphotyrosyl bond (22,24), classifying it in the type I 5Ј topoisomerase family as proposed by Roca (38). Sequence analysis further showed that it is a single polypeptide containing putative helicase and topoisomerase domains located in the amino-and carboxy-terminal, respectively, parts of the protein (9). The helicase domain exhibits motifs found in DNA and RNA helicases, and the topoisomerase domain exhibits a significant similarity with the 5Ј topoisomerase I (protein ) from Escherichia coli. From all of these data, a mechanism of reverse gyration involving the concerted action of two such domains was proposed (9, 14).To date, four other archaeal reverse gyrase genes have been sequenced: from Sulfolobus shibatae (21), Methanopyrus kandleri (26), Pyrococcus furiosus (3), and Methanococcus jannaschii (7). A comparative analysis of reverse gyrases from two members of the order Sulfolobales (S. acidocaldarius and S. shibatae) and M. kandleri with the other type I topoisomerases of the 5Ј family (21) showed that the reverse gyrases constitute a new group within this family distinct from the previously described TopA and TopB groups, represent...