Regulatory Circuits Controlling Photosynthesis Gene Expression

Much progress has been made of late in identifying transcription factors that control synthesis of the bacterial photosystem. Perhaps most surprising is the overlapping nature (Figure 3Figure 3) of the individual circuits that together form a remarkably complex “regulatory network”. These circuits are responsible for coordinating synthesis of bacteriochlorophyll and carotenoids with that of the light harvesting and reaction center structural polypeptides that bind the photopigments. Balanced synthesis of these components occurs not only during static growth conditions where expression patterns are at fixed levels, but also during derepressing and repressing shifts in growth conditions.Figure 3Complexity of the Photosynthesis Regulatory NetworkSuperimposing the three regulatory circuits (as described in Figure 2Figure 2) on the overlapping transcription units of the superoperons emphasizes the complexities involved in coordinating expression of photopigment biosynthesis genes with those of the light harvesting and reaction center structural genes.View Large Image | View Hi-Res Image | Download PowerPoint SlideIn addition to genes involved in photosynthesis, the RegB-RegA regulon depicted in Figure 3Figure 3 includes genes involved in CO2 fixation (cbb; Qian and Tabita, 1995) and electron transport (cycA; Eraso and Kaplan 1994xEraso, J.M and Kaplan, S. J. Bacteriol. 1994; 176: 32–43PubMedSee all ReferencesEraso and Kaplan 1994). It is entirely possible that other nonphotosynthetic metabolic functions known to be affected by anaerobiosis are also regulated by this circuit. For example, profound changes in the cell ultrastructure, including the formation of intracytoplasmic vesicles that house the photosystem, occur when cultures are shifted from aerobic to photosynthetic growth conditions. It is therefore reasonable to assume that the synthesis of specific lipids (known to be induced upon anaerobiosis) may be controlled by the photosynthesis regulatory network. One of the remaining challenges will be to determine how the cell coordinately regulates synthesis of these very different cellular processes.Given the complexity of coordinating synthesis of photosystem components it is likely that additional regulatory factors will soon be discovered. For example, unless RegA proves to be a novel transcription regulator, it appears that one or more downstream component(s) within the RegB-RegA phosphorylation circuit activates transcription of the puf, puh, and puc operons. Inspection of the puf and puh promoter sequences also indicate that these promoters are probably recognized by an alternative σ subunit (designated σP) which has not yet been identified (Bauer 1995xSee all ReferencesBauer 1995). Mutational analyses also indicate that expression of bacteriochlorophyll and carotenoid genes are regulated by unidentified anaerobic transactivators in addition to aerobic repression by CrtJ (14xMa, D, Cook, D.N, O'Brien, D.A, and Hearst, J.E. J. Bacteriol. 1993; 175: 2037–2045PubMedSee all References, 17xPonnampalam, S, Buggy, J.J, and Bauer, C.E. J. Bacteriol. 1995; 177: 2990–2997PubMedSee all References). Regulated expression of the puc promoter also appears to involve a homolog of the E. coli transcription factor IHF and is influenced by several distant cis-acting sites (Lee et al. 1993xLee, J.K, Wang, S, Eraso, J.M, Gardner, J, and Kaplan, S. J. Biol. Chem. 1993; 268: 24491–24497PubMedSee all ReferencesLee et al. 1993). These outstanding questions suggest that the diagram in Figure 3Figure 3 represents only a rudimentary sketch of the circuits that control synthesis of the bacterial photosystem and that there is much work to be done before we will have a detailed understanding of the molecular mechanisms whereby environmental signals and energy demands of the cell can affect its physiology.