Light-induced division and genomic synchrony in phototrophically growing cultures of Rhodopseudomonas sphaeroides

Assembly of photosynthetic apparatus in Rhodobacter sphaeroides as revealed by functional assessments at different growth phases and in synchronized and greening cells

The development of photosynthetic membranes of intact cells of Rhodobacter sphaeroides was tracked by light-induced absorption spectroscopy and induction and relaxation of the bacteriochlorophyll fluorescence. Changes in membrane structure were induced by three methods: synchronization of cell growth, adjustment of different growth phases and transfer from aerobic to anaerobic conditions (greening) of the bacteria. While the production of the bacteriochlorophyll and carotenoid pigments and the activation of light harvesting and reaction center complexes showed cell-cycle independent and continuous increase with characteristic lag phases, the accumulation of phospholipids and membrane potential (electrochromism) exhibited stepwise increase controlled by cell division. Cells in the stationary phase of growth demonstrated closer packing and tighter energetic coupling of the photosynthetic units (PSU) than in their early logarithmic stage. The greening resulted in rapid (within 0-4 h) induction of BChl synthesis accompanied with a dominating role for the peripheral light harvesting system (up to LH2/LH1 ~2.5), significantly increased rate (~7·10(4) s(-1)) and yield (F v/F max ~0.7) of photochemistry and modest (~2.5-fold) decrease of the rate of electron transfer (~1.5·10(4) s(-1)). The results are discussed in frame of a model of sequential assembly of the PSU with emphasis on crowding the LH2 complexes resulting in an increase of the connectivity and yield of light capture on the one hand and increase of hindrance to diffusion of mobile redox agents on the other hand.

Stringency in the absence of ppGpp accumulation in Rhodobacter sphaeroides

Leucine deprivation of either phototrophically or chemotrophically growing cells of Rhodobacter sphaeroides resulted in a restriction in the continued accumulations of cellular RNA, phospholipids, and protein. Phototrophically growing cells also displayed restrictions in the accumulations of cellular carotenoids and bacteriochlorophyll. Leucine deprivation, however, did not provoke the accumulation of cellular ppGpp or alter the steady-state levels of ppGpp, ATP, or GTP in cells of R. sphaeroides.

Cell-cycle-specific oscillation in the composition of chromatophore membrane in Rhodospirillum rubrum

Synchrony in phototrophic cultures of Rhodospirillum rubrum was induced by stationary-phase cycling or by alterations in light intensity. Intracytoplasmic chromatophore membranes were prepared by differential centrifugation. Analysis of the composition of chromatophores obtained from cells at different times indicated that the protein/bacteriochlorophyll a ratio was constant throughout the cell cycle but that the protein/phospholipid ratio oscillated. This cell-cycle-dependent fluctuation in chromatophore membrane composition was reflected in the buoyant densities of the isolated chromatophores.

Broad-host-range plasmid vector for the in vitro construction of transcriptional/translational lac fusions

A broad-host-range plasmid was constructed that allows the in vitro formation of beta-galactosidase fusions. DNA from the photosynthetic bacterium Rhodopseudomonas sphaeroides was cloned into this plasmid and a number of R. sphaeroides isolates were recovered that had varying levels of beta-galactosidase activity. beta-galactosidase antigenic activity from the fusion strains could be localized immunologically in polypeptides with an Mr of 120 000 or greater. Expression of beta-galactosidase activity under control of fusion derivatives was either very low or nonexistent in Escherichia coli relative to R. sphaeroides, indicating that R. sphaeroides promoters or translational start signals function poorly in E. coli.

Cell-cycle-specific biosynthesis of the photosynthetic membrane of Rhodopseudomonas sphaeroides. Structural implications

Structural changes association with the intracytoplasmic membrane during the cell cycle of the photosynthetic bacterium Rhodopseudomonas sphaeroides have been studied by freeze-fracture electron microscopy. The isolated intracytoplasmic membrane vesicles, chromatophores, were fused in order to obtain large fracture faces, allowing more precise measurements and statistical analysis of both intramembrane particle density and size determinations. The intramembrane particle density of the protoplasmic face (PF) of the intracytoplasmic membrane, (from 4970 to 8290/micrometers 2), was shown to be a linear function of the protein/phospholipid ratio (from 2.5 to 5.1, w/w) of the intracytoplasmic membrane. Under constant light intensity, both the average particle size and particle size distribution remained unchanged during the cell cycle. These results provide the structural basis for the earlier reported cell-cycle-specific variations in both protein/phospholipid ratio and alternation in phospholipid structure of the intracytoplasmic membrane of R. sphaeroides during photosynthetic growth. The average particle diameter in the PF face of the intracytoplasmic membrane was 8.25, 9.08 and 9.75 nm at incident light intensities of 4000, 500 and 30 ft X cd, respectively. When chromatophores were fused with small, unilamellar liposomes, the intramembrane particle density decreased as input liposome phospholipid increased, whereas the particle size remained constant and particle distribution became random.

In vivo metabolic intermediates of phospholipid biosynthesis in Rhodopseudomonas sphaeroides

The in vivo metabolic pathways of phospholipid biosynthesis in Rhodopseudomonas sphaeroides have been investigated. Rapid pulse-chase-labeling studies indicated that phosphatidylethanolamine and phosphatidylglycerol were synthesized as in other eubacteria. The labeling pattern observed for N-acylphosphatidylserine (NAPS) was inconsistent with the synthesis of this phospholipid occurring by direct acylation of phosphatidylserine (PS). Rather, NAPS appeared to be kinetically derived from an earlier intermediate such as phosphatidic acid or more likely CDP-diglyceride. Tris-induced NAPS accumulation specifically reduced the synthesis of PS. Treatment of cells with a bacteriostatic concentration of hydroxylamine (10 mM) greatly reduced total cellular phospholipid synthesis, resulted in accumulation of PS, and stimulated the phosphatidylglycerol branch of phospholipid metabolism relative to the PS branch of the pathway. When the cells were treated with a lower hydroxylamine dosage (50 microM), total phospholipid synthesis lagged as PS accumulated, however, phospholipid synthesis resumed coincident with a reversal of PS accumulation. Hydroxylamine alone was not sufficient to promote NAPS accumulation but this compound allowed continued NAPS accumulation when cells were grown in medium containing Tris. The significance of these observations is discussed in terms of NAPS biosynthesis being representative of a previously undescribed branch of the phospholipid biosynthetic sequence.

Light-mediated regulation of phospholipid synthesis in Rhodopseudomonas sphaeroides

The relationship between the culture levels of guanosine-5'-diphosphate-3'-diphosphate (ppGpp) and the rates of synthesis and accumulation of cellular phospholipids was examined in cultures of Rhodopseudomonas sphaeroides that had been subjected to immediate decreases in incident light intensity. After a high-to-low light transition of high-light-adapted cells, an immediate inhibition of total cellular phospholipid production occurred coincident with a rapid accumulation of culture ppGpp. The inhibition of phospholipid accumulation occurred at the level of phospholipid synthesis rather than turnover, and both the extent of ppGpp accumulation and the degree of inhibition of phospholipid synthesis were directly dependent upon the magnitude of the light transition. Maximum inhibition (greater than 90%) of the rate of cellular phospholipid synthesis occurred after transitions from 5,350 to 268 1x and lower, including transitions to the dark, with comparable inhibition being exerted upon the rates of synthesis of individual species of phospholipids. Reinitiation of culture phospholipid accumulation in cultures shifted from 5,350 to 1,070 1x and lower occurred 65 to 70 min subsequent to the downshift in light intensity, apparently irrespective of the culture level of ppGpp.

Long-chain fatty acid assimilation By rhodopseudomonas sphaeroides

Exogenously supplied long-chain fatty acids have been shown to markedly alleviate the inhibition of phototrophic growth of cultures of Rhodopseudomonas sphaeroides caused by the antibiotic cerulenin. Monounsaturated and polyunsaturated C18 fatty acids were most effective in relieving growth inhibition mediated by cerulenin. Medium supplementation with saturated fatty acids (C14 to C18) failed to influence the inhibitory effect of cerulenin. The addition of mixtures of unsaturated and saturated fatty acids to the growth medium did not enhance the growth of cerulenin-inhibited cultures above that obtained with individual unsaturated fatty acids as supplements. Resolution and fatty acid analysis of the extractable lipids of R. sphaeroides revealed that exogenously supplied fatty acids were directly incorporated into cellular phospholipids. Cells treated with cerulenin displayed an enrichment in their percentage of total saturated fatty acids irrespective of the presence of exogenous fatty acids. Cerulenin produced comparable inhibitions of the rates of both fatty acid and phospholipid synthesis and was further found to preferentially inhibit unsaturated fatty acid synthesis.

Light-dependent regulation of the synthesis of soluble and intracytoplasmic membrane proteins of Rhodopseudomonas sphaeroides

Cells of Rhodopseudomonas sphaeroides grown under saturating light conditions (30 W/m2) and then shifted to low light intensity (3 W/m2) required 2.5 h to adapt to the new lower light conditions. After the shift, cell growth, whole cell protein accumulation, and bacteriochlorophyll accumulation ceased immediately. Approximately midway into the adaptation period, bacteriochlorophyll synthesis commenced at a new, higher rate, which continued through the beginning of the low-light growth period until new steady-state levels were reached. Immediately after the downshift, the rate of cellular protein synthesis declined to 22% of its preshift rate. Pulse-labeling of protein throughout the adaptation period and comparison with a steady-state prelabel culture revealed that synthesis of two of the three light-harvesting proteins, as well as two additional high-molecular-weight photosynthetic membrane proteins, was derepressed three- to fivefold compared with bulk cellular protein. Finally, the synthesis of at least three soluble proteins showed light-dependent regulation after the light downshift. These results are discussed in terms of the light-dependent regulation of synthesis of the photosynthetic membrane macromolecular components and the division of protein synthesis between the photosynthetic membranes and the soluble cell phase.