Regulatory roles of spnT, a novel gene located within transposon TnTIR

The transposon TnTIR contains spnIR quorum-sensing system regulating sliding motility and the production of nuclease, biosurfactant, and prodigiosin in Serratia marcescens. Within TnTIR, a gene named spnT is upstream of and co-transcribed with spnI. SpnT is a cytoplasmic protein and its level peaks during early stationary phase. spnT over-expression resulted in inhibition of sliding motility and synthesis of prodigiosin, and biosurfactant similar to spnR. spnT but not spnR over-expression induced cell elongation and aberrant DNA replication in S. marcescens and Escherichia coli strains. In comparison with wild-type E. coli strain, over-expression of spnT in an E. coli priA and dnaC double-mutant strain did not lead to the aberrant cell morphology phenotypes, suggesting SpnT may act through the recombination-dependent DNA replication system. As spnT over-expression inhibited swarming but not swimming motility, SpnT may indirectly function as a negative regulator of surface-dependent migration and secondary metabolite production.