Light-responsive gene expression in cyanobacteria.

The cyanobacteria constitute a kingdom of the domain Bacteria (44) whose members manifest a rich diversity of habitats and metabolisms. Species differ with respect to possession of a panoply of specializations, a sampling of which includes diazotrophy, filamentous growth form, production of buoyancy organelles, and differentiation of specialized cell types (5, 47). However, the group can be defined by the unifying property of chlorophyll a-based, oxygenic photosynthesis (57). This uniquely cyanobacterial photoautotrophism is the source of the Earth’s oxidizing atmosphere (51) and extends to plants, whose chloroplasts trace their ancestry to this kingdom (44, 57). Given the central role of photosynthesis in the metabolism of cyanobacteria, it is not surprising that the light environment directly regulates expression of genes that encode key components of the photosynthetic apparatus. The two structures whose light-responsive regulation has been best studied are the photosystem II (PSII) complex itself and its closely associated light-harvesting antenna, the phycobilisome. Genes that encode components of both complexes are likely to be under the control of wavelength-specific photoreceptors that initiate light-responsive signal transduction pathways. Regulation of genes encoding the PSII reaction center proteins. At the heart of the PSII reaction center is a dimer of two structurally related proteins designated D1 and D2, each of which contributes ligands to the cofactors that mediate primary photochemistry (1). The genes that encode these two proteins, psbA and psbD, are conserved in plants, where they reside as single genes in the chloroplast genome. In contrast, cyanobacteria have one (25) to four psbA genes and two psbD genes (21). The regulation of the psbA and psbD genes is best understood for Synechococcus sp. strain PCC 7942, a unicellular cyanobacterium which has three psbA genes that encode two distinct forms of the D1 protein. The three genes are regulated differentially in response to changes in light intensity. In the laboratory, this is demonstrated by a lowto high-light shift equivalent to the increase in light intensity that a cell in shallow water might experience when a cloud moves away from blocking the sun. At low light, greater than 80% of the psbA transcripts are from psbAI, and the only D1 protein detectable in the thylakoid membrane is form I, the product of this gene (13, 32). Immediately upon a shift to high light, the psbAII and psbAIII genes are induced, and the psbAI message is actively degraded (34). After 30 min of exposure, the psbAII message makes up 80% of the psbA transcript population, and the identical products of this message and that of psbAIII, collectively known as form II, are substituted for form I in the thylakoid (13, 32). Several lines of evidence suggest that the two forms of D1 are functionally distinct and that form II provides a selective advantage at high light (12, 13, 32, 33). The D2 polypeptide is encoded by a two-member psbD family, one gene of which is subject to regulation by light (6). The dicistronic psbDI-psbC operon encodes D2 and CP43, a chlorophyll a-binding protein of PSII. It is expressed with little variation at all light intensities. The monocistronic psbDII gene is expressed at very low levels when cells are at low light intensity but is rapidly induced when the light intensity increases. Because the two psbD genes encode identical polypeptides, light-responsive psbD expression causes quantitative changes in D2 production rather than a qualitative interchange like that of the two forms of D1. The regulatory events seem to be important for physiology: growth of a mutant which lacks an active psbDII gene is impaired at high light, and its thylakoids sampled at high light show a deficiency in D2 (6). Intensity (photon flux density) and spectral quality are interdependent characteristics of light which are not completely separable. Although light-responsive expression of the psbA and psbD genes has been studied during shifts in intensity of white light, recent experiments revealed that the characteristic lowto high-light shift responses are induced by low-fluence blue light (56). Like the phenomena associated with complementary chromatic adaptation (CCA) described below, this appears to be a chromatically reversible response, with red light canceling an inductive blue signal. Therefore, subtle shifts in the blue/red ratio may provide information about changes in the light environment that are linked to light intensity. The physiology of PSII suggests that intensity may be the important parameter, even if the photoreception system is set up to work through light quality. The rate of photosynthetic electron transport is proportional to light intensity up to a point, but plants and cyanobacteria enter a state known as photoinhibition under very high light intensity or under other conditions in which light intensity exceeds the capacity of the cell to transport electrons (such as limitations in CO2 or specific ions) (46, 59). The prevailing model for the source of photoinhibition is that the D1 polypeptide is specifically damaged during photochemistry and that it must be cleaved, removed from the PSII complex, and replaced to maintain photosynthetic electron transport. When the rate of damage exceeds that of synthesis of new D1 subunits, photoinhibition results (42, 43, 52, 58). The changes in psbA and psbD expression and in PSII protein synthesis and turnover match well with predictions based on a need for increased synthesis of the reaction center subunits at high light (6, 31, 32). It is not yet clear whether the blue/red and low/high-light responses of the psbA family in Synechococcus sp. result from the same, independent, or overlapping sensory pathways (56). Unusual cis elements in the untranslated leader regions of the psbA genes. A series of facts suggested that transcriptional regulation accounts for the rapid increases in Synechococcus sp. psbAII and psbAIII messages at high light (7). 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