During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

Muscle-eye-brain disease (MEB) is an autosomal recessive disorder characterized by congenital muscular dystrophy, ocular abnormalities, and lissencephaly. Mammalian O-mannosyl glycosylation is a rare type of protein modification that is observed in a limited number of glycoproteins of brain, nerve, and skeletal muscle. Here we isolated a human cDNA for protein O-mannose beta-1,2-N-acetylglucosaminyltransferase (POMGnT1), which participates in O-mannosyl glycan synthesis. We also identified six independent mutations of the POMGnT1 gene in six patients with MEB. Expression of most frequent mutation revealed a great loss of the enzymatic activity. These findings suggest that interference in O-mannosyl glycosylation is a new pathomechanism for muscular dystrophy as well as neuronal migration disorder.

To evaluate the physiological significance of cyclic electron f low around photosystem (PS) I, we used a reverse genetic approach to focus on 11 chloroplast genes that encode homologs of mitochondrial complex I subunits (ndhA-K). Since their discovery, the exact function of the respiratory components in plant chloroplasts has been a matter of discussion. We disrupted one of these genes (ndhB) in tobacco by chloroplast transformation. Analysis of the transient increase in chlorophyll f luorescence after actinic light illumination and the redox kinetics of P700 (reaction center chlorophylls of PS I) suggest that the cyclic electron f low around PS I is impaired in the ndhB-deficient transformants. Transformants grew normally in a greenhouse, suggesting that the cyclic electron f low around PS I mediated by ndh gene products is dispensable in tobacco under mild environmental conditions.Photosynthetic electron flow provides the first stable products of photosynthesis: NADPH and ATP. Despite the importance of this electron flow, a fundamental problem remains unsolved; that is, how an appropriate balance between the production of NADPH and ATP is maintained. To answer this question, the contributions of the Q cycle, cyclic electron flow around photosystem (PS) I, and pseudocyclic electron flow (water-water cycle) in chloroplast energetics must be evaluated quantitatively (1). There is little doubt that cyclic electron flow around PS I provides extra ATP in some cellular processes, such as N 2 fixation in cyanobacterial heterocysts (2) and CO 2 concentration in cyanobacterial and C4 photosynthesis (3-8). However, it is unclear whether this cyclic electron flow contributes to the supply of ATP during steady-state photosynthesis in nonspecialized photosynthetic cells of higher plants (1, 9, 10).Although molecular biological dissection using a reverse genetic approach is an effective means to evaluate the physiological significance of cyclic electron flow around PS I, it has not been attempted because of a lack of information about the genes responsible for the electron flow. However, the discovery of an ndhB-deficient mutant of Synechocystis PCC6803 that lacked cyclic electron flow around PS I led to the idea that electron f low is mediated by the respiratory complex, NAD(P)H dehydrogenase, in cyanobacteria (4-8).Eleven ndh genes encoding homologs of mitochondrial complex I subunits are also present in the chloroplast genome of higher plants (11,12). Although respiratory function is limited to the mitochondria, a respiratory complex, NAD(P)H dehydrogenase, may catalyze cyclic electron flow around PS I in chloroplasts, as in cyanobacteria. However, the existence of NAD(P)H dehydrogenase-mediated electron flow in higher plants is still a matter of controversy (1, 13), because genes for the crucial flavoprotein subunits have not yet been identified (14). Moreover, physiological evidence alone has been insufficient to show an NAD(P)H dehydrogenase-mediated pathway for cyclic electron flow around PS I in higher plants...