Schizosaccharomyces pombe rec mutants were previously isolated on the basis of their deficiency in meiotic recombination at the ade6 locus. We surveyed their meiotic recombination deficiencies at and between other loci. In reclO mutants recombinant frequencies in the ~2-Mb region surrounding the ade6 locus were reduced 10-to 100-fold, but recombinant frequencies at or between nine other unlinked loci were reduced <3-fold. The reclO mutations are recessive and are on chromosome I; the ade6 region is on chromosome III. These results indicate that the reclO gene product is required for activation of meiotic recombination in the ~2-Mb region surrounding ade6 but not in the other regions surveyed. Similar ade6 regional specificities were observed for rec8 and recll. We infer that there are multiple activators of meiotic recombination, each specific for a limited set of loci, and we discuss how these regional activators may work. The frequency of homologous recombination is not uniform across chromosomes but is controlled by a complex interplay of special sites and gene products. Control sites have been identified in both prokaryotes and eukaryotes, and in some cases the proteins acting at them have been elucidated (Smith 1988). Recombination can also be locally influenced by other chromosomal processes such as transcription (Voelkel-Meiman et al. 1987;Thomas and Rothstein 1989). In all of these cases, the effects are limited to about a few genes or several kilobases from the control site. We report here that the products of the rec8, reclO, and recll genes (Rec8, ReclO, and Recll) of Schizosaccharomyces pombe are required for high meiotic recombinant frequencies in a region covering about one-half (~2 Mb) of chromosome III; the reductions of recombinant frequencies by the rec8, reclO, and recll mutations are 10-to 100-fold greater in this region than in the other regions examined. These results imply recombination regulatory factors acting only in parts of the genome and over considerably larger distances than in previously reported cases.Meiotic recombination-deficient [rec] mutants of S. pombe were isolated by screening for deficiency of recombination between chromosomal and plasmid-bome alleles of the ade6 locus (Ponticelli and Smith 1989;DeVeaux et al. 1992). They were subsequently shown to be deficient for recombination between homologous chroPiesent address: 1527 143td Avenue SE, Tenino, Washington 98589 USA.mosomes. The 39 recessive mutations analyzed defined 16 complementation groups spread throughout the genome.The rec mutations have a wide range of effects on meiotic recombination (Ponticelli and Smith 1989;DeVeaux et al. 1992). At the ade6 locus, recombinant frequencies are reduced from 3-to 1000-fold, depending on the rec gene mutated. When intragenic recombination (at the ura4 locus) and intergenic recombination (between the pro2 and arg3 loci) were tested, the recombinant frequencies in some rec mutants were reduced much less than those at ade6. These differential reductions might have been attributable...

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Recent evidence has implicated single-stranded DNA-binding protein (SSB) expression level as an important factor in microbial radiation resistance. The genome of the extremely radiation resistant bacterium Deinococcus radiodurans contains genes for two SSB homologs: the homodimeric, canonical Ssb, encoded by the gene ssb, and a novel pentameric protein encoded by the gene ddrB. ddrB is highly induced upon exposure to radiation, and deletions result in decreased radiation-resistance, suggesting an integral role of the protein in the extreme resistance exhibited by this organism. Although expression of ssb is also induced after irradiation, Ssb is thought to be involved primarily in replication. In this study, we demonstrate that Ssb in D. radiodurans is essential for cell survival. The lethality of an ssb deletion cannot be complemented by providing ddrB in trans. In addition, the radiation-sensitive phenotype conferred by a ddrB deletion is not alleviated by providing ssb in trans. By altering expression of the ssb gene, we also show that lower levels of transcription are required for optimal growth than are necessary for high radiation resistance. When expression is reduced to that of E. coli, ionizing radiation resistance is similarly reduced. UV resistance is also decreased under low ssb transcript levels where growth is unimpaired. These results indicate that the expression of ssb is a key component of both normal cellular metabolism as well as pathways responsible for the high radiation tolerance of D. radiodurans.

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Planetary protection is governed by the Outer Space Treaty and includes the practice of protecting planetary bodies from contamination by Earth life. Although studies are constantly expanding our knowledge about life in extreme environments, it is still unclear what the probability is for terrestrial organisms to survive and grow on Mars. Having this knowledge is paramount to addressing whether microorganisms transported from Earth could negatively impact future space exploration. The objectives of this study were to identify cultivable microorganisms collected from the surface of the Mars Science Laboratory, to distinguish which of the cultivable microorganisms can utilize energy sources potentially available on Mars, and to determine the survival of the cultivable microorganisms upon exposure to physiological stresses present on the martian surface. Approximately 66% (237) of the 358 microorganisms identified are related to members of the Bacillus genus, although surprisingly, 22% of all isolates belong to non-spore-forming genera. A small number could grow by reduction of potential growth substrates found on Mars, such as perchlorate and sulfate, and many were resistant to desiccation and ultraviolet radiation (UVC). While most isolates either grew in media containing ‡10% NaCl or at 4°C, many grew when multiple physiological stresses were applied. The study yields details about the microorganisms that inhabit the surfaces of spacecraft after microbial reduction measures, information that will help gauge whether microorganisms from Earth pose a forward contamination risk that could impact future planetary protection policy.