Control of bacteriochlorophyll accumulation by light in Rhodobacter capsulatus

Abstract

The regulation of bacteriochlorophyll and carotenoid synthesis in purple non-sulfur photosynthetic bacteria has been the subject of intense investigation for over a quarter century. Cohen-Bazire et al. (6), studying Rhodobacter sphaeroides and Rhodospirillum rubrum, demonstrated that oxygen and light prevented the accumulation ofboth bacteriochlorophyll and carotenoids. They hypothesized that the two photopigments were governed by the oxidation state of some component of the electron transport chain. This was based on the assumption that the same mechanism governed regulation by both oxygen and light. Much more recently, Arnheim and Oelze (1) measured intracellular bacteriochlorophyll levels in chemostat cultures with carefully controlled oxygen and light levels. They found that light acted independently of oxygen in the regulation of bacteriochlorophyll accumulation. Further investigation revealed that while both oxygen and light control bacteriochlorophyll accumulation, aminolevulinate synthase responds only to changes in oxygen tension, not to changes in light intensity (11).Two studies have focused on the control of bacteriochlorophyll formation by oxygen in Rhodobacter capsulatus. Clark et al. (4) followed the accumulation of bch gene mRNA after a shift from high to low oxygen tension. They found that transcription of the bch genes increased when the culture was shifted to 2% oxygen. Using bch-lacZ fusions, Biel and Marrs (2) found that transcription of the bch genes increased two to fourfold when the oxygen tension was lowered from 23 to 3%. However, they did not detect a change in bch gene transcription when the light intensity was changed. It was suggested that the lack of response to light was due to the inability of the fusion strains to synthesize bacteriochlorophyll and thus the pigment-protein complexes. This study has focused on the control of bacteriochlorophyll accumulation by light in R. capsulatus growing under low aeration. Exposure to bright light did not reduce transcription from the bch genes, nor did it reduce carbon flow to bacteriochlorophyllide a. These results suggest that in cultures grown under low oxygen, bright light inhibits bacteriochlorophyll accumulation, but does not reduce bacteriochlorophyll synthesis. MATERIALS AND METHODSBacterial strains. The bacterial strains used in this work are described in Table 1. Strain AJB516 was isolated by introducing the conjugative plasmid pRPS404 (10) into strain Y142 and purifying a crtD puf recombinant. Strain AJB522 is a spontaneous bchA' revertant of strain BY1651.Media and growth conditions. R. capsulatus strains used to measure bacteriochlorophyll accumulation were grown overnight in a malate-minimal salts (RCV) medium (18) and subcultured into RCV medium supplemented with 0.6% glucose, 0.5% pyruvate, and 0.5 M dimethyl sulfoxide (RCV+) to yield an initial density of 25 Klett units (red filter). The culture tubes and sparging apparatus have been described previously (2). The cultures were sparged with a mixture of 92% nitrogen-5% carbon...

Isolation of a Rhodobacter capsulatus mutant that lacks c-type cytochromes and excretes porphyrins

“…Most of the attention has been devoted to regulation by oxygen tension (6,8,15,39), light intensity (1,5,23), DNA supercoiling (40), and the effects of the recently discovered pufQ gene (3,16). Relatively little attention has been paid to the interrelationships between heme and bacteriochlorophyll syntheses.…”

Posttranscriptional regulation by light of the steady-state levels of mature B800-850 light-harvesting complexes in Rhodobacter capsulatus

“…No free Bchl is detected in normal cells of photosynthetic bacteria, so the ratio of pigment-binding peptides to Bchl must be equal to or greater than 1. It has been well established that the total Bchl content of anaerobically grown R. capsulatus increases as the light intensity decreases (1,3,7,8,12,17); therefore, when cells are shifted from a high-to a low-light environment, more Bchl is produced, which could bind to preexistent components of a B800-850 apopeptide pool. These protein-pigment complexes would be more resistant to proteolytic degradation and could then function in light absorption.…”

Oxygen-Mediated Regulation of Porphobilinogen Formation inRhodobacter capsulatus

“…The protoporphyrin IX concentration was determined by comparing the fluorescence (excitation wavelength ϭ 407 nm, emission wavelength ϭ 605 nm) with a standard curve developed using commercially obtained protoporphyrin IX (Porphyrin Products, Logan, Utah). The protein concentration of cells extracted with acetone-methanol was determined as previously described (4). Due to day-to-day variations in porphyrin levels of the inoculum and growth conditions, all measurements reported are representative values from several trials.…”