The tetrapyrrole biosynthetic pathway and its regulation in Rhodobacter capsulatus

Abstract

The purple anoxygenic photosynthetic bacterium Rhodobacter capsulatus is capable of growing in aerobic or anaerobic conditions, in the dark or using light, etc. Achieving versatile metabolic adaptations from respiration to photosynthesis requires the use of tetrapyrroles such as heme and bacteriochlorophyll, in order to carry oxygen, to transfer electrons, and to harvest light energy. A third tetrapyrrole, cobalamin (vitamin B(12)), is synthesized and used as a cofactor for many enzymes. Heme, bacteriochlorophyll, and vitamin B(12) constitute three major end products of the tetrapyrrole biosynthetic pathway in purple bacteria. Their respective synthesis involves a plethora of enzymes, several that have been characterized and several that are uncharacterized, as described in this review. To respond to changes in metabolic requirements, the pathway undergoes complex regulation to direct the flow of tetrapyrrole intermediates into a specific branch(s) at the expense of other branches of the pathway. Transcriptional regulation of the tetrapyrrole synthesizing enzymes by redox conditions and pathway intermediates is reviewed. In addition, we discuss the involvement of several transcription factors (RegA, CrtJ, FnrL, AerR, HbrL, Irr) as well as the role of riboswitches. Finally, the interdependence of the tetrapyrrole branches on each other synthesis is discussed.

Fig 1

Overview of the tetrapyrrole biosynthesis pathway with its substrates, major intermediates and products. Number of enzyme reactions for each branch is indicated.

Fig 2

Overview of the transcriptional regulation of the tetrapyrrole biosynthesis pathway. Transcription factors involved are RegA (○, based on gene expression studies; ⌷ based on both gene expression and DNA binding studies), FnrL (◊, repressor; ◈, activator), AerR (⊗), CrtJ (★), HbrL (⊥). Arrows indicate a feedback control by heme operated through CrtJ or HbrL. Genes identified from bioinformatics analysis only, with no experimental studies, are specified with a question mark.