Regulatory Circuits Controlling Photosynthesis Gene Expression

Carl E. Bauer and Terry H. Bird reduced (and protonated) it migrates to the cytochrome bc1 complex. The cytochrome bc1 complex then cycles Department of Biology Indiana University electrons from the quinone back to the reaction center via cytochrome c2 (or cytochrome cy). The bc1 complex Bloomington, Indiana 47405 also functions to translocate protons across the cytoplasmic membrane which creates a proton gradient used to generate ATP. Few organisms can match the remarkable versatility in energy metabolism exhibited by purple photosynthetic The Photosynthesis Gene Cluster bacteria. Many of these organisms can obtain cellular All essential genetic information needed to synthesize energy from light, inorganic compounds, or organic a “core” bacterial photosystem, composed of light harcompounds depending on the chemical and physical vesting-I and reaction center complexes, is located on conditions of the environment. Although this flexibility a 46 kb region of the R. capsulatus chromosome known permits purple bacteria to thrive in very different habias the “photosynthesis gene cluster” (Figure 1; Alberti et tats, it also requires that each mode of energy metabolism be regulated so as to prevent unnecessary biosynthesis of alternative bioenergetic systems. This is particularly true for photosynthesis where synthesis of the photosystem consumes large quantities of energy. The ability of bacteria to regulate synthesis of a photosystem was initially observed in 1887 by Erwin Esmarch who described the first successful isolation of a photosynthetic bacterium in pure culture. In his essay, he noted that “the spirillum only produces pigment if the air is completely or almost completely excluded from the colony” (a review of this work has recently been published by Gest, 1995). This observation was later extended to include light regulation of the photosystem in the classic study of Cohen-Bazire et al. (1957). Contemporary studies involving Rhodobacter capsulatus have revealed a remarkably complex circuitry that regulates expression of photosystem genes in response to alterations in oxygen tension and light intensity. This minireview briefly describes components of the R. capsulatus photosystem and summarizes what is currently known about the overlapping regulatory circuits.