Analysis of rpoS mRNA in Salmonella dublin: Identification of multiple transcripts with growth-phase-dependent variation in transcript stability

In Salmonella dublin, rpoS encodes an alternative sigma factor of the RNA p olymerase that activates a variety of stationary-phase-induced genes, inclu ding some virulence-associated genes. In this work, we studied the regulati on and transcriptional organization of rpoS during growth. We found two tra nscripts, 2.3 and 1.6 kb in length, that represent the complete rpoS sequen ce, The 2.3-kb transcript is a polycistronic message that also includes the upstream nlpD gene. It is driven by a weak promoter with increasing activi ty when cells enter early stationary growth. The 1.6-kb message includes 56 6 bp upstream of the rpoS start codon, It is transcribed from a strong sigm a(70) RNA polymerase-dependent promoter which is independent of growth. The decay of this transcript decreases substantially in early stationary growt h, resulting in a significant net increase in rpoS mRNA levels. These level s are approximately 10-fold higher than the levels of the 2.3-kb mRNA, indi cating that the 1.6-kb message is mainly responsible for RpoS upregulation, In addition to the 2.3- and 1.6-kb transcripts, two smaller 1.0- and 0.4-k b RNA species are produced from the nlpD-rpoS locus. They do not allow tran slation of full-length RpoS; hence their significance for rpoS regulation r emains unclear, We conclude that of four transcripts arising from the nlpD- rpoS locus, only one plays a significant role in rpoS expression in S. dubl in. Its upregulation when cells enter stationary growth is due primarily to an increase in transcript stability.