Mutation in a gene encoding anti-σ factor in A. brasilense confers tolerance to elevated temperature, antibacterial peptide and PEG-200 via carotenoid synthesis

Abstract

Azospirillum brasilense Sp7 has been shown to overproduce carotenoids if the anti-σ factor (anti-σE)-encoding gene is inactivated. The anti-σ mutant (Car-1) of A. brasilense Sp7 was more tolerant to the stresses generated by elevated temperature (40°C), PEG-200 (30 mg mL -1) and the antibacterial agent Polymyxin-B (PMB, 25 μg mL -1) but not to elevated salinity (15 mg mL-1). Inhibition of carotenoid synthesis by diphenylamine inhibited the ability of the mutant to tolerate all the three stresses. Out of the four stress agents, only elevated temperature and salinity induced the rpoE promoter and increased the carotenoid content in Sp7 as well as in the Car-1 mutant. Comparison of the membrane permeability of the parent and the mutant by a PMB-N-phenyl-1-naphthylamine coupled assay showed that the presence of carotenoids in the mutant reduced the permeability of their membranes. Our study indicates that the carotenoid synthesis, which is under the control of extracytoplasmic function σ factor (σE) in A. brasilense Sp7, plays a positive role in tolerating elevated temperature, the antibacterial peptide and PEG-200. © 2008 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.