Many photosynthetic bacteria use inorganic sulfur compounds as electron donors for carbon dioxide fixation. A thiosulfate-induced cytochrome c has been purified from the photosynthetic ␣-proteobacterium Rhodovulum sulfidophilum. This cytochrome c 551 is a heterodimer of a diheme 30-kDa SoxA subunit and a monoheme 15-kDa SoxX subunit. The cytochrome c 551 structural genes are part of an 11-gene sox locus. Sequence analysis suggests that the ligands to the heme iron in SoxX are a methionine and a histidine, while both SoxA hemes are predicted to have unusual cysteine-plus-histidine coordination. A soxA mutant strain is unable to grow photoautotrophically on or oxidize either thiosulfate or sulfide. Cytochrome c 551 is thus essential for the metabolism of both these sulfur species. Periplasmic extracts of wild-type R. sulfidophilum exhibit thiosulfate: cytochrome c oxidoreductase activity. However, such activity can only be measured for a soxA mutant strain if the periplasmic extract is supplemented with purified cytochrome c 551 . Gene clusters similar to the R. sulfidophilum sox locus can be found in the genome of a green sulfur bacterium and in phylogenetically diverse nonphotosynthetic autotrophs.

The two major chemoreceptors of Escherichia coli, Tsr and Tar, mediate opposite responses to the same changes in cytoplasmic pH (pH i ). We set out to identify residues involved in pH i sensing to gain insight into the general mechanisms of signaling employed by the chemoreceptors. did not change the polarity but altered the time course of pH i response. These results suggest that the electrostatic properties of a short cytoplasmic region within the linker region that connects the second transmembrane helix to the first methylation helix is critical for switching the signaling state of the chemoreceptors during pH sensing. Similar conformational changes of this region in response to external ligands may be critical components of transmembrane signaling.Many biological processes, such as enzyme reactions and interactions between proteins, are influenced by pH. Therefore, cells have to sense and adapt to changes in extracellular and intracellular pH. Despite the accumulated knowledge about pH-dependent regulation in a wide variety of organisms, the molecular mechanisms of pH sensing are still poorly understood.Behavioral responses of Escherichia coli and Salmonella typhimurium to changes in pH provide a convenient system for studying the pH-sensing mechanism. These bacteria show repellent responses to weak acids and attractant responses to weak bases (1, 2). These responses are generated by decreases and increases of cytoplasmic pH (pH i ). 1 The changes in pH i were documented by 31 P nuclear magnetic resonance spectroscopy (3, 4). Usually, pH i in E. coli is maintained at around 7.5 over a range of extracellular pH (pH o ) values from 5.0 to 9.0 (3, 5). However, this strong pH i homeostasis can be disrupted by the addition of weak acids or weak bases to the culture medium. When pH o is lower than pH i , weak acids can traverse the membrane in their protonated (uncharged) form and release protons in the cytoplasm to decrease pH i . Similarly, when pH o is higher than pH i , weak bases can traverse the membrane in deprotonated (uncharged) forms and capture protons in the cytoplasm to increase pH i . These changes in pH i correlate well with tactic responses to weak acids and weak bases (4).The signal transduction pathway for chemotaxis in E. coli and S. typhimurium has been extensively studied at the molecular level (for reviews, see Refs. 6 -9). These organisms have a set of related methyl-accepting chemoreceptors that includes the serine receptor Tsr and the aspartate receptor Tar. These receptors have a remarkable ability to sense a variety of stimuli, including chemoattractants, chemorepellents, temperature, and pH.Tar and, presumably, the other chemoreceptors exist as a homodimer of about 60-kDa subunits (10). The dimeric cytoplasmic domains form stable complexes with the histidine kinase CheA and the adaptor protein CheW (11,12). Furthermore, the receptors, together with the CheA and CheW proteins, cluster at a cell pole (13).CheA phosphorylates itself and then serves as a phosphodonor for the response regul...

The use of transvaginal ultrasound-guided embryo transfer significantly improved both pregnancy and implantation rates. Although technically difficult, we suggest its use may maximize the chances of achieving a successful pregnancy outcome.

Genes coding for putative RegA, RegB, and SenC homologues were identified and characterized in the purple nonsulfur photosynthetic bacteria Rhodovulum sulfidophilum and Roseobacter denitrificans, species that demonstrate weak or no oxygen repression of photosystem synthesis. This additional sequence information was then used to perform a comparative analysis with previously sequenced RegA, RegB, and SenC homologues obtained from Rhodobacter capsulatus and Rhodobacter sphaeroides. These are photosynthetic bacteria that exhibit a high level of oxygen repression of photosystem synthesis controlled by the RegA-RegB two-component regulatory system. The response regulator, RegA, exhibits a remarkable 78.7 to 84.2% overall sequence identity, with total conservation within a putative helix-turn-helix DNA-binding motif. The RegB sensor kinase homologues also exhibit a high level of sequence conservation (55.9 to 61.5%) although these additional species give significantly different responses to oxygen. A Rhodovulum sulfidophilum mutant lacking regA or regB was constructed. These mutants produced smaller amounts of photopigments under aerobic and anaerobic conditions, indicating that the RegA-RegB regulon controls photosynthetic gene expression in this bacterium as it does as in Rhodobacter species. Rhodobacter capsulatus regA-or regB-deficient mutants recovered the synthesis of a photosynthetic apparatus that still retained regulation by oxygen tension when complemented with reg genes from Rhodovulum sulfidophilum and Roseobacter denitrificans. These results suggest that differential expression of photosynthetic genes in response to aerobic and anaerobic growth conditions is not the result of altered redox sensing by the sensor kinase protein, RegB.

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