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Characterization of Two Methanopterin Biosynthesis Mutants of Methylobacterium extorquens AM1 by Use of a Tetrahydromethanopterin Bioassay. Madeline E. Rasche, 2004.An enzymatic assay was developed to measure tetrahydromethanopterin [H4MPT] levels in wild-type and mutant cells of Methylobacterium extorquens AM1 . H4MPT was detectable in wild-type cells but not in strains with a mutation of either the orf4 or the dmrA gene, suggesting a role for these two genes in H4MPT biosynthesis.The protein encoded by orf4 catalyzed the reaction of ribofuranosylaminobenzene5'-phosphate synthase, the first committed step of H4MPT biosynthesis.These results provide the first biochemical evidence for H4MPT biosynthesis genes in bacteria. Posttranscriptional Control of Quorum-Sensing-Dependent Virulence Genes by DksA in Pseudomonas aeruginosa. Florence Jude, 2003.Pseudomonas aeruginosa controls the secretion of extracellular virulence factors, including rhamnolipids and LasB elastase, by the las and rhl quorum-sensing systems . Here, we mutated the dksA gene of P . aeruginosa by insertion of an  -Hg
cassette . The mutant displayed growth rates similar to that of the
wild type in rich medium but was impaired in growth in defined
minimal medium . Production of rhamnolipids and LasB elastase by the
dksA mutant was only 4 and 10%, respectively, of wild-type levels .
These defects could be partially complemented by introduction
of the plasmid-encoded dksA genes from P . aeruginosa or
Escherichia coli . In the dksA mutant, the expression of
rhlI was increased early during exponential growth, but
expression of other quorum-sensing regulator genes—lasR,
lasI, and rhlR—was not affected . Although the
transcription of the lasB and rhlAB genes was comparable
between the dksA mutant and the wild-type strain in peptone
tryptic soy broth medium, we observed reduced translation of
both genes in the dksA mutant . Similarly, we found that full
translation of lasB and rhlAB genes in E . coli also
requires the dksA gene . DksA is therefore a novel regulator
involved in the posttranscriptional control of extracellular
virulence factor production in P . aeruginosa .
A Novel Family of Escherichia coli Toxin-Antitoxin Gene Pairs. Jason M. Brown, 2003.Bacterial toxin-antitoxin protein pairs (TA pairs) encode a toxin protein, which poisons cells by binding and inhibiting an essential enzyme, and an antitoxin protein, which binds the toxin and restores viability . We took an approach that did not rely on sequence homology to search for unidentified TA pairs in the genome of Escherichia coli K-12 . Of 32 candidate genes tested, ectopic expression of 6 caused growth inhibition . In this report, we focus on the initial characterization of yeeV, ykfI, and ypjF, a novel family of toxin proteins . Coexpression of the gene upstream of each toxin restored the growth rate to that of the uninduced strain . Unexpectedly, we could not detect in vivo protein-protein interactions between the new toxin and antitoxin pairs . Instead, the antitoxins appeared to function by causing a large reduction in the level of cellular toxin protein . Are Readily Culturable Bacteria in Coastal North Sea Waters Suppressed by Selective Grazing Mortality?. Christine Beardsley, 2003.We studied the growth of six culturable bacterial lineages from coastal North Sea picoplankton in environmental samples under different incubation conditions . The grazing pressure of heterotrophic nanoflagellates (HNF) was reduced either by double prefiltration through 0.8-µm-pore-size filters or by 10-fold dilutions with 0.2-µm (pore-size) prefiltered seawater . We hypothesized that those  -proteobacterial genera that are rapidly enriched would also be most strongly affected by HNF regrowth . In the absence of HNF, the mean protein content per bacterial cell increased in both treatments compared to environmental samples, whereas the opposite trend was found in incubations of unaltered seawater . Significant responses to the experimental manipulations were observed in Alteromonas, Pseudoalteromonas, and Vibrio populations . No treatment-specific effects could be detected for members of the Roseobacter group, the Cytophaga latercula-C . marinoflava lineage, or the NOR5 clade . Statistical analysis confirmed a transient increase in the proportions of Alteromonas, Pseudoalteromonas, and Vibrio cells at reduced HNF densities only, followed by an overproportional decline during the phase of HNF regrowth . Cells from these genera were significantly larger than the community average in the dilution treatments, and changes in their relative abundances were negatively correlated with HNF densities . Our findings suggest that bacteria affiliated with frequently isolated genera such as Alteromonas, Pseudoalteromonas, and Vibrio might be rare in coastal North Sea picoplankton because their rapid growth response to changing environmental conditions is counterbalanced by a higher grazing mortality .
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