Molecular Analysis and Organization of the B Operon in Staphylococcus aureus

Department of Medical Microbiology, University of Zürich, Switzerland.
Journal of Bacteriology (Impact Factor: 2.81). 01/2006; 187(23):8006-19. DOI: 10.1128/JB.187.23.8006-8019.2005
Source: PubMed


The alternative sigma factor σB of Staphylococcus aureus controls the expression of a variety of genes, including virulence determinants and global regulators. Genetic manipulations
and transcriptional start point (TSP) analyses showed that the sigB operon is transcribed from at least two differentially controlled promoters: a putative σA-dependent promoter, termed sigBp1, giving rise to a 3.6-kb transcript covering sa2059-sa2058-rsbU-rsbV-rsbW-sigB, and a σB-dependent promoter, sigBp3, initiating a 1.6-kb transcript covering rsbV-rsbW-sigB. TSP and promoter-reporter gene fusion experiments indicated that a third promoter, tentatively termed sigBp2 and proposed to lead to a 2.5-kb transcript, including rsbU-rsbV-rsbW-sigB, might govern the expression of the sigB operon. Environmental stresses, such as heat shock and salt stress, induced a rapid response within minutes from promoters
sigBp1 and sigBp3. In vitro, the sigBp1 promoter was active in the early growth stages, while the sigBp2 and sigBp3 promoters produced transcripts throughout the growth cycle, with sigBp3 peaking around the transition state between exponential growth and stationary phase. The amount of sigB transcripts, however, did not reflect the concentration of σB measured in cell extracts, which remained constant over the entire growth cycle. In a guinea pig cage model of infection,
sigB transcripts were as abundant 2 and 8 days postinoculation as values found in vitro, demonstrating that sigB is indeed transcribed during the course of infection. Physical interactions between staphylococcal RsbU-RsbV, RsbV-RsbW,
and RsbW-σB were inferred from a yeast (Saccharomyces cerevisiae) two-hybrid approach, indicating the presence of a partner-switching mechanism in the σB activation cascade similar to that of Bacillus subtilis. The finding that overexpression of RsbU was sufficient to trigger an immediate and strong activation of σB, however, signals a relevant difference in the regulation of σB activation between B. subtilis and S. aureus in the cascade upstream of RsbU.

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    • "Furthermore, we detected a decrease in the expression level of the alkaline shock protein 23 (asp23) gene. In S. aureus,asp23 transcription is typically used as a marker for the activity of the alternative sigma factor Sigma B[35], which responds to several stress conditions by transiently increasing its activity[36,37], suggesting that kendomycin exerts a negative effect on Sigma B activity. We further investigated the effect of kendomycin on the expression levels of two additional genes that encode proteins involved in the oxidative stress response in S. aureus, KatA and alkyl hydroperoxide reductase (AhpC)[38], which are regulated by the peroxide response regulator PerR[39]. "
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    • "The S. aureus sigma B operon resembles that of the homologous B. subtilis operon. It contains σB, an anti-σB factor RsbW, an anti-anti-σB factor RsbV, and RsbU, a Mn2+-dependent phosphatase that positively controls σB activity by dephosphorylating RsbV [80], [81]. The sigma B regulon includes genes directly up-regulated by σB and genes indirectly regulated via σB-dependent expression of regulatory factors such as the SarA transcription factor [76], [77]. "
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    • "Under stress conditions, RsbV is dephosphorylated by one or more specific PP2C-type phosphatases, resulting in the sequestration of RsbW and the activation of σB. This part of this regulatory is conserved in Bacilli (van Schaik et al., 2005), Staphylococcus aureus (Palma and Cheung, 2001; Senn et al., 2005; Pané-Farré et al., 2006) and L. monocytogenes (Wiedmann et al., 1998; Ferreira et al., 2004). However, there are considerable differences in the upstream part of the σB activation pathway (Ferreira et al., 2004; van Schaik et al., 2004a), reflecting differences in the mechanisms of stress sensing and signaling in the various bacteria. "
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