Oxygen tension and nutrient starvation are major signals that regulate agfD promoter activity and expression of the multicellular morphotype in Salmonella typhimurium

Expression of multicellular behaviour (rdar morphotype) is a characteristic of wild-type Salmonella typhimurium strains. The key target for the regula tion of rdar morphotype expression is the agfD promoter. The regulation of two rdar morphotypes, regulated and semi-constitutive (the latter differs f rom the former by the insertion of A after position -17), by various enviro nmental conditions was studied using transcriptional fusions to the regulat ed and semi-constitutive agfD promoters by Western blot analysis and phenot ypic analysis of the rdar morphotype. AgfD promoter activities were strongl y dependent on oxygen tension. Expression maxima were observed in rich medi um under microaerophilic conditions and in minimal medium under aerobic con ditions. The regulated rdar morphotype was only expressed under conditions of maximal promoter activity. Glucose did not influence rdar morphotype exp ression, and the two promoters showed no consistent response to pH. In the stationary phase of growth, nitrogen and phosphate depletion were found to be signals that switch on the agfD promoters. In the logarithmic phase of g rowth, ethanol was the stress signal that enhanced rdar morphotype expressi on. The results indicate that, although the regulated and semi-constitutive agfD promoters are key factors in the grade of expression of the multicell ular behaviour, common signals such as oxygen tension, depletion of nutrien ts and ethanol vary their levels of expression significantly.