Nuclease overexpression mutants of Serratia marcescens

Phosphate limitation induces the intergeneric inhibition of Pseudomonas aeruginosa by Serratia marcescens isolated from paper machines

Phosphate is an essential nutrient for heterotrophic bacteria, affecting bacterioplankton in aquatic ecosystems and bacteria in biofilms. However, the influence of phosphate limitation on bacterial competition and biofilm development in multispecies populations has received limited attention in existing studies. To address this issue, we isolated 13 adhesive bacteria from paper machine aggregates. Intergeneric inhibition of Pseudomonas aeruginosa WW5 by Serratia marcescens WW4 was identified under phosphate-limited conditions, but not in Luria–Bertani medium or M9 minimal medium. The viable numbers of the pure S. marcescens WW4 culture decreased over 3 days in the phosphate-limited medium; however, the mortality of S. marcescens WW4 was significantly reduced when it was co-cultured with P. aeruginosa WW5, which appeared to sustain the S. marcescens WW4 biofilm. In contrast, viable P. aeruginosa WW5 cells immediately declined in the phosphate-limited co-culture. To identify the genetic/inhibitory element(s) involved in this process, we inserted a mini-Tn5 mutant of S. marcescens WW4 that lacked inhibitory effect. The results showed that an endonuclease bacteriocin was involved in this intergeneric inhibition by S. marcescens WW4 under phosphate limitation. In conclusion, this study highlights the importance of nutrient limitation in bacterial interactions and provides a strong candidate gene for future functional characterisation.

Identification of Rgg-regulated exoproteins of Streptococcus pyogenes

Streptococcus pyogenes secretes many proteins that influence host-pathogen interactions. Despite their importance, relatively little is known about the regulation of these proteins. The rgg gene (also known as ropB) is required for the expression of streptococcal erythrogenic toxin B (SPE B), an extracellular cysteine protease that contributes to virulence. Proteomics was used to determine if rgg regulates the expression of additional exoproteins. Exponential- and stationary-phase culture supernatant proteins made by S. pyogenes NZ131 rgg and NZ131 speB were separated by two-dimensional electrophoresis. Differences were identified in supernatant proteins from both exponential- and stationary-phase cultures, although considerably more differences were detected among stationary-phase supernatant proteins. Forty-two proteins were identified by peptide fingerprinting with matrix-assisted laser desorption mass spectrometry. Mitogenic factor, DNA entry nuclease (open reading frame [ORF 226]), and ORF 953, which has no known function, were more abundant in the culture supernatants of the rgg mutant compared to the speB mutant. ClpB, lysozyme, and autolysin were detected in the culture supernatant of the speB mutant but not the rgg mutant. To determine if Rgg affected protein expression at the transcriptional level, real-time (TaqMan) reverse transcription (RT)-PCR was used to quantitate Rgg-regulated transcripts from NZ131 wild-type and speB and rgg mutant strains. The results obtained with RT-PCR correlated with the proteomic data. We conclude that Rgg regulates the transcription of several genes expressed primarily during the stationary phase of growth.

Cleavage experiments with deoxythymidine 3',5'-bis-(p-nitrophenyl phosphate) suggest that the homing endonuclease I-PpoI follows the same mechanism of phosphodiester bond hydrolysis as the non-specific Serratia nuclease

We show here that two nucleases, Serratia nuclease and I-PpoI, with contrasting specificities, i.e. non-specific vs. highly sequence specific, share a structurally similar active site region with conservation of the catalytically relevant histidine and asparagine residues. On the basis of a comparison of the available structures and biochemical data for wild type and mutant variants of Serratia nuclease and I-PpoI we propose that both enzymes have a common catalytic mechanism, a proposition that is supported by our finding that both enzymes accept deoxythymidine 3',5'-bis-(p-nitrophenyl phosphate) as a substrate and cleave it in an identical manner. According to this mechanism a histidine residue functions as a general base and Mg2+ bound to an asparagine residue as a Lewis acid in phosphodiester bond cleavage.