FIG 6.

Proposed network of alternative sigma factors regulating the expression of the crtE2 gene of the carotenoid biosynthetic pathway in A. brasilense Sp7. In response to appropriate stimuli, RpoE1 regulates RpoH2 and RpoH5, while RpoE2 regulates RpoH1 and RpoH4. RpoH1 and RpoH2 initiate the transcription of a common gene, crtE2, to regulate carotenoid biosynthesis in A. brasilense. The crtE1 promoter is also activated in an RpoE1-dependent manner but via an unknown intermediate regulator or regulators. Thick arrows indicate the cascade established in this study by mutation studies as well as by using a two-plasmid system. Thin arrows show the cascades inferred from promoter activity assays using the two-plasmid system. The dotted arrow indicates an indirect regulation of crtE1 by RpoE1 via an unknown intermediate sigma factor or factors.