FIG. 4.

Bchl and heme biosynthetic pathways. The relevant genes, the regulation of which is either established or predicted on the basis of sequence analyses, are depicted. The question mark to the right of a regulator symbol indicates that regulation by the corresponding regulator is inferred from sequence analyses and is therefore not proven experimentally. The Bchl biosynthetic branch is shown in a yellow box. To the left of the yellow box, the regulation of four inferred operons which are involved in Bchl biosynthesis is represented. The red and blue arrows represent induction under oxygen-limiting conditions (semiaerobic or anaerobic conditions) and repression under high-oxygen conditions, respectively. The outer membrane TspO protein affects the expression of PpsR target genes by acting on HemN posttranscriptionally (24). At the bottom, the regulation of the puc and puf operons as well as the puhA gene, which encode the structural polypeptides of the reaction center and light-harvesting complexes, is presented. Abbreviations: MV, monovinyl; DV, divinyl; CoA, coenzyme A.