Our group previously identified a defective rho mutant (SP3710) in a Tn5 mutagenesis screen of C. crescentus for mutants with
decreased tolerance to NaCl. Tn5 insertion before the first codon in the amino terminal RNA-binding motif results in the expression of a 45-kDa carboxyl-terminal domain of Rho, expressed by a transposon promoter (Italiani et al., 2002; Italiani & Marques, 2005). The sensitivity of the mutant to bicyclomycin suggests that the ATP-binding site of Rho is intact in the 45-kDa truncated protein (Italiani & Marques, 2005). Moreover, the Rho protein in strain SP3710 is functional enough to ensure viability. However, its transcription termination activity is severely impaired, as observed by the lack of autoregulation (Italiani & Marques, 2005). The studies on Rho function in C. crescentus reported here Epigenetics inhibitor are based on our observation that rho mutant strain SP3710 shows an unusual distortion in its response to environmental stress (Italiani et al., 2002). Strain SP3710 is sensitive to NaCl, as expected from the screen used for Cytoskeletal Signaling inhibitor its isolation. However, this rho mutant strain is essentially wild type in its response to UV light and alkaline pH and is only moderately sensitive to acid pH and to heat shock. In contrast, strain SP3710 is highly sensitive to exogenously added hydrogen peroxide (H2O2), both in the exponential and
in the stationary phase. Although a variety of cellular phenotypes have been reported for rho mutants, to our knowledge, strain SP3710 is the first rho mutant with such drastic
distortions in its stress response. Thus, strain SP3710 and the partially functional Rho it expresses are new and potentially valuable tools for identifying additional physiological roles of rho. Caulobacter crescentus has several enzymes involved in the oxidative stress response. It was shown to express a cytosolic iron superoxide dismutase (FeSOD), a periplasmic copper–zinc 4-Aminobutyrate aminotransferase superoxide dismutase (CuZnSOD) and a catalase–peroxidase (KatG) (Schnell & Steinman, 1995; Steinman et al., 1997). Caulobacter crescentus contains just one bifunctional catalase–peroxidase, KatG, and evidently lacks monofunctional catalases and thiol peroxidases. In this work, our goal was to identify the determinants of C. crescentus oxidative stress response affected by the rho mutation, based on the oxidative stress phenotype of strain SP3710 cited above and prior studies on the roles and regulation of antioxidant defense enzymes in C. crescentus (Schnell & Steinman, 1995; Steinman et al., 1997; Rava et al., 1999; Alvarez-Martinez et al., 2006). Caulobacter crescentus strain NA1000 (Evinger & Agabian, 1977) was used as the wild type in all the experiments; strain SP3710 has a Tn5 insertion in the rho gene (Italiani et al., 2002; Italiani & Marques, 2005) and strain SGC111 is a katG null mutant (Steinman et al., 1997).