Salmonella typhimurium displays cyclical patterns of sensitivity to UV-C killing during prolonged incubation in the stationary phase of growth.

Stationary phase cells of Salmonella typhimurium were more resistant to killing by UV-C irradiation than those from the exponential phase. Analysis of the tolerance of cells taken at different stages of prolonged incubation as batch cultures to 60 or 100 J m(2) doses of UV-C revealed cycles of resistance and tolerance. The possible involvement of rpoS-controlled functions in mediating these cycles could be discounted because they were also detected in an rpoS minus mutant of S. typhimurium. The results are discussed in the context of heterogeneity in cells of stationary phase cultures of S. typhimurium.

Nudix hydrolases that degrade dinucleoside and diphosphoinositol polyphosphates also have 5-phosphoribosyl 1-pyrophosphate (PRPP) pyrophosphatase activity that generates the glycolytic activator ribose 1,5-bisphosphate.

A total of 17 Nudix hydrolases were tested for their ability to hydrolyze 5-phosphoribosyl 1-pyrophosphate (PRPP). All 11 enzymes that were active toward dinucleoside polyphosphates with 4 or more phosphate groups as substrates were also able to hydrolyze PRPP, whereas the 6 that could not and that have coenzyme A, NDP-sugars, or pyridine nucleotides as preferred substrates did not degrade PRPP. The products of hydrolysis were ribose 1,5-bisphosphate and P(i). Active PRPP pyrophosphatases included the diphosphoinositol polyphosphate phosphohydrolase (DIPP) subfamily of Nudix hydrolases, which also degrade the non-nucleotide diphosphoinositol polyphosphates. K(m) and k(cat) values for PRPP hydrolysis for the Deinococcus radiodurans DR2356 (di)nucleoside polyphosphate hydrolase, the human diadenosine tetraphosphate hydrolase, and human DIPP-1 (diadenosine hexaphosphate and diphosphoinositol polyphosphate hydrolase) were 1 mm and 1.5 s(-1), 0.13 mm and 0.057 s(-1), and 0.38 mm and 1.0 s(-1), respectively. Active site mutants of the Caenorhabditis elegans diadenosine tetraphosphate hydrolase had no activity, confirming that the same active site is responsible for nucleotide and PRPP hydrolysis. Comparison of the specificity constants for nucleotide, diphosphoinositol polyphosphate, and PRPP hydrolysis suggests that PRPP is a significant substrate for the D. radiodurans DR2356 enzyme and for the DIPP subfamily. In the latter case, generation of the glycolytic activator ribose 1,5-bisphosphate may be a new function for these enzymes.

nuvA, an Aspergillus nidulans gene involved in DNA repair and recombination, is a homologue of Saccharomyces cerevisiae RAD18 and Neurospora crassa uvs-2.

A 40 kb genomic clone and 2.3 kb EcoRI subclone that rescued the DNA repair and recombination defects of the Aspergillus nidulans nuvA11 mutant were isolated and the subclone sequenced. The subclone hybridized to a cosmid in a chromosome-specific library confirming the assignment of nuvA to linkage group IV and indicating its closeness to bimD. Amplification by PCR clarified the relative positions of nuvA and bimD. A region identified within the subclone, encoding a C3HC4 zinc finger motif, was used as a probe to retrieve a cDNA clone. Sequencing of this clone showed that the nuvA gene has an ORF of 1329 bp with two introns of 51 bp and 60 bp. Expression of nuvA appears to be extremely low. The putative NUVA polypeptide has two zinc finger motifs, a molecular mass of 48906 Da and has 39% identity with the Neurospora crassa uvs-2 and 25% identity with the Saccharomyces cerevisiae RAD18 translation products. Although mutations in nuvA, uvs-2 and RAD18 produce similar phenotypes, only the nuvA11 mutation affects meiotic recombination. A role for nuvA in both DNA repair and genetic recombination is proposed.

R391: a conjugative integrating mosaic comprised of phage, plasmid, and transposon elements.

The conjugative, chromosomally integrating element R391 is the archetype of the IncJ class of mobile genetic elements. Originally found in a South African Providencia rettgeri strain, R391 carries antibiotic and mercury resistance traits, as well as genes involved in mutagenic DNA repair. While initially described as a plasmid, R391 has subsequently been shown to be integrated into the bacterial chromosome, employing a phage-like integration mechanism closely related to that of the SXT element from Vibrio cholerae O139. Analysis of the complete 89-kb nucleotide sequence of R391 has revealed a mosaic structure consisting of elements originating in bacteriophages and plasmids and of transposable elements. A total of 96 open reading frames were identified; of these, 30 could not be assigned a function. Sequence similarity suggests a relationship of large sections of R391 to sequences from Salmonella, in particular those corresponding to the putative conjugative transfer proteins, which are related to the IncHI1 plasmid R27. A composite transposon carrying the kanamycin resistance gene and a novel insertion element were identified. Challenging the previous assumption that IncJ elements are plasmids, no plasmid replicon was identified on R391, suggesting that they cannot replicate autonomously.