Figure 1.

Effects of multicopy iraP and deletion of iraP on σ^S stability. The half-life of σ^S was measured in cells grown at 37°C in LB. Protein synthesis was inhibited with chloramphenicol at OD₆₀₀ of ∼0.3 and ∼3 for the determination of σ^S half-life in exponential phase and stationary phase, respectively. Samples were removed at specific time points and analyzed by immunoblot with an anti-σ^S antiserum. (A) σ^S half-life was determined in the strain CRB316 (P_BAD-rpoS990’-‘lacZ chromosomal fusion) containing either pHDB3, as vector control, or pIraP. The accumulation and the degradation of σ^S-LacZ are shown in the top panel and chromosomally-encoded σ^S in the bottom panel. Quantification of the σ^S signals is shown in the graph. The density for samples at time 0 was set to 100% for each of the chase experiments. Half-lives (t_½) were calculated by regression analysis of the exponential decay of σ^S. (B) Effect of iraP expression from a foreign inducible promoter on σ^S stability in the strain AB009 (ΔiraP∷kn) carrying either the vector control pP_LlacO or pP_LlacO-iraP. The cells were grown until exponential phase in the presence or absence of 1 mM IPTG, as indicated. (C) Comparison of σ^S half-lives in the ΔiraP (AB006) and wild-type (MG1655) strains.