Journal of Bacteriology, February 1999, p. 1264-1268, Vol. 181, No. 4
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Institute of Microbiology,
Received 8 June 1998/Accepted 25 November 1998
In Salmonella dublin, rpoS encodes an
alternative sigma factor of the RNA polymerase that activates a variety
of stationary-phase-induced genes, including some virulence-associated
genes. In this work, we studied the regulation and transcriptional
organization of rpoS during growth. We found two
transcripts, 2.3 and 1.6 kb in length, that represent the complete
rpoS sequence. 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 activity when cells enter
early stationary growth. The 1.6-kb message includes 566 bp upstream of
the rpoS start codon. It is transcribed from a strong
70 RNA polymerase-dependent promoter which is
independent of growth. The decay of this transcript decreases
substantially in early stationary growth, resulting in a significant
net increase in rpoS mRNA levels. These levels are
approximately 10-fold higher than the levels of the 2.3-kb mRNA,
indicating 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-kb RNA species are produced from the
nlpD-rpoS locus. They do not allow translation of
full-length RpoS; hence their significance for rpoS
regulation remains unclear. We conclude that of four transcripts
arising from the nlpD-rpoS locus, only one plays a significant role in rpoS expression in S. dublin. Its upregulation when cells enter stationary growth is
due primarily to an increase in transcript stability.
*
Corresponding author. Mailing address: Institute of
Microbiology, Swiss Federal Institute of Technology, ETH-Zentrum,
Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland. Phone:
41-1-632-33-53. Fax: 41-1-632-11-48. E-mail:
paesold{at}micro.biol.ethz.ch.
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