Instability of bacteriophage lambda initiator O and P proteins in DNA replication

Prophage LambdaSo uses replication interference to suppress reproduction of coexisting temperate phage MuSo2 in Shewanella oneidensis MR-1

Many bacterial genomes carry multiple prophages that compete with each other, potentially affecting the physiology, fitness, and pathogenicity of their hosts. However, molecular mechanisms of such prophage-prophage conflicts remain poorly understood. The genome of Shewanella oneidensis MR-1, a Gammaproteobacterium residing in aquatic environments and notable for its ability to reduce metal ions, harbours four prophages, two of which (LambdaSo and MuSo2) form infectious virions during biofilm formation. Here, we constructed indicator strains of LambdaSo and MuSo2 by deleting the corresponding prophages from the MR-1 chromosome and investigated their reproduction. Interestingly, the fitness of MuSo2 increased in the absence of LambdaSo, suggesting that prophage LambdaSo repressed MuSo2 reproduction. Partial deletion of LambdaSo from the MR-1 chromosome revealed that gene cluster R of LambdaSo, which was responsible for the switch to the lytic cycle and LambdaSo genome replication initiation, was necessary and sufficient to repress MuSo2. Furthermore, activation of cluster R genes facilitated replication of cluster R-encoding DNA and inhibited host and MuSo2 DNA replication. These findings suggest that LambdaSo represses MuSo2 propagation by inhibiting DNA replication during simultaneous induction. We predict that such a mechanism of inter-prophage interference is more widespread in bacteria than currently appreciated.

Lambda gpP-DnaB Helicase Sequestration and gpP-RpoB Associated Effects: On Screens for Auxotrophs, Selection for Rif(R), Toxicity, Mutagenicity, Plasmid Curing

The bacteriophage lambda replication initiation protein P exhibits a toxic effect on its Escherichia coli (E. coli) host, likely due to the formation of a dead-end P-DnaB complex, sequestering the replicative DnaB helicase from further activity. Intracellular expression of P triggers SOS-independent cellular filamentation and rapidly cures resident ColE1 plasmids. The toxicity of P is suppressed by alleles of P or dnaB. We asked whether P buildup within a cell can influence E. coli replication fidelity. The influence of P expression from a defective prophage, or when cloned and expressed from a plasmid was examined by screening for auxotrophic mutants, or by selection for rifampicin resistant (Rif(R)) cells acquiring mutations within the rpoB gene encoding the β-subunit of RNA polymerase (RNAP), nine of which proved unique. Using fluctuation assays, we show that the intracellular expression of P evokes a mutator effect. Most of the Rif(R) mutants remained P(S) and localized to the Rif binding pocket in RNAP, but a subset acquired a P(R) phenotype, lost sensitivity to ColE1 plasmid curing, and localized outside of the pocket. One P(R) mutation was identical to rpo*Q148P, which alleviates the UV-sensitivity of ruv strains defective in the migration and resolution of Holliday junctions and destabilizes stalled RNAP elongation complexes. The results suggest that P-DnaB sequestration is mutagenic and supports an earlier observation that P can interact with RNAP.

Phage Lambda P protein: trans-activation, inhibition phenotypes and their suppression

The initiation of bacteriophage λ replication depends upon interactions between the oriλ DNA site, phage proteins O and P, and E. coli host replication proteins. P exhibits a high affinity for DnaB, the major replicative helicase for unwinding double stranded DNA. The concept of P-lethality relates to the hypothesis that P can sequester DnaB and in turn prevent cellular replication initiation from oriC. Alternatively, it was suggested that P-lethality does not involve an interaction between P and DnaB, but is targeted to DnaA. P-lethality is assessed by examining host cells for transformation by ColE1-type plasmids that can express P, and the absence of transformants is attributed to a lethal effect of P expression. The plasmid we employed enabled conditional expression of P, where under permissive conditions, cells were efficiently transformed. We observed that ColE1 replication and plasmid establishment upon transformation is extremely sensitive to P, and distinguish this effect from P-lethality directed to cells. We show that alleles of dnaB protect the variant cells from P expression. P-dependent cellular filamentation arose in ΔrecA or lexA[Ind^-] cells, defective for SOS induction. Replication propagation and restart could represent additional targets for P interference of E. coli replication, beyond the oriC-dependent initiation step.

Nitric oxide from L-arginine stimulates the soluble guanylate cyclase in adrenal glands

The formation of nitric oxide (NO) by an L-arginine:NO synthase and its stimulation of the soluble guanylate cyclase was studied in rat whole adrenal and bovine cortex and medulla cytosol. In the presence of L-arginine, the stimulation of soluble guanylate cyclase was accompanied by the formation of citrulline and NO2-, formed from NO. The NO synthase was NADPH- and Ca(2+)-dependent and was inhibited by several L-arginine analogues. These results indicate that rat and bovine adrenal cytosol contains an L-arginine:NO synthase.