Novel pleiotropic regulators of flotation, secondary metabolite production, and virulence in Serratia sp. ATCC 39006
Serratia sp. ATCC 39006 (S39006) is a Gram-negative, rod-shaped enterobacterium known for the production two antibiotics; the β-lactam, 1-carbapen-2-em-3-carboxylic acid (a carbapenem) and the red-pigmented tripyrrole, 2-methyl-3-pentyl-6-methoxyprodigiosin (prodigiosin; a prodiginine). It also produces plant cell wall degrading enzymes (PCWDEs) and is capable of flagellar-mediated swimming and swarming motility. S39006 is the only enterobacterium known to naturally produce gas vesicles (GVs). GVs are proteinaceous, intracellular organelles that increase the buoyancy of a cell and enable flotation upwards through the water column and colonisation of air-liquid interfaces. The production and regulation of GVs is a complex process with a range of regulatory inputs. GVs are expressed from a cluster of 19 genes arranged in two contiguous operons, gvpA1-gvpY and gvrA-gvrC. Prior to this study, three regulators encoded within the GV cluster, which are essential for GV production, had been described: GvrA, GvrB, and GvrC. Other regulators that have been identified include the post-transcriptional regulator RsmA, the repressor of the ribose operon, RbsR, and the DeoR-family transcriptional regulator, FloR. This study employed random transposon mutagenesis, visual screening of mutant phenotypes, cloning and sequencing, and bioinformatic analysis to identify novel regulators of GV production. This included mutants with transposon insertions in genes encoding an O-antigen ligase (waaL), the sigma factor σ54 (rpoN), and a transcription factor (dksA). The waaL mutant exhibited increased transcription and expression of GV genes but was unable to float. Pleiotropic effects of the transposon insertion included an increase in carbapenem production and a decrease in motility and virulence in a Caenorhabditis elegans model. The rpoN mutant showed a reduction in GV and pectate lyase production, swimming and swarming motility, and an increase in carbapenem production. The dksA mutant showed a decrease in GV and antibiotic production, motility, and virulence in C. elegans. A quantitative proteomic analysis was undertaken comparing the rpoN and dksA mutants against wild type S39006 to understand their regulatory roles. Expression of proteins involved in GV biogenesis, antibiotic production and motility as well as previously identified transcriptional regulators were significantly altered in each mutant, compared to wild type.
Serratia, gas vesicle, carbapenem, prodigiosin, rpoN, dksA
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