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Journal of Bacteriology, May 2002, p. 2815-2820, Vol. 184, No. 10
0021-9193/02/$04.00+0 DOI: 10.1128/JB.184.10.2815-2820.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Biology, Indiana University, Bloomington, Indiana 47405
Received 7 August 2001/ Accepted 24 January 2002
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FIG. 1. A model of the electron transport system and ATP synthase in the R. capsulatus membrane. Energy production in the form of ubihydroquinone (UQH2) occurs in the photosystem reaction center. Ubiquinol oxidase utilizes UQH2 to reduce oxygen to water, while cytochrome cbb3 oxidase obtains reducing power from cytochrome c2 or cy via the cytochrome bc1 complex. Electron transfer is coupled to proton translocation at the sites indicated. Protons can reenter the cell via ATP synthase to make ATP. Electrons also shuttle back to the reaction center via cytochrome c2 or cy.
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Ubiquinol oxidase expression. Expression of ubiquinol oxidase was assayed using a translational lacZ fusion to the first gene in the cydAB operon that contains 920 bp of DNA upstream from cydA (21). This plasmid was mobilized as described previously (24) into regA (19), crtJ (20), hvrA (3), aerR (20), and fnrL (20) single-mutant strains, as well as regA-crtJ (20) and regA-fnrL (20) double-mutant strains. Each of the constructed strains was tested for ß-galactosidase activity under aerobic, semiaerobic, and anaerobic (photosynthetic) growth conditions as reported by Buggy and Bauer (2).
The expression pattern observed for ubiquinol oxidase in wild-type R. capsulatus was similar to that reported by Swem et al. (21) (Fig. 2). Specifically, expression was lowest under aerobic conditions, intermediate under anaerobic conditions (1.8-fold higher), and highest (3.2-fold higher) under semiaerobic growth conditions. The effect of a disruption of regA was also similar to that reported by Swem et al. (21), in which expression was significantly lower than for the wild type (by 81 to 87%) under all tested growth conditions.
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FIG. 2. ß-Galactosidase analysis of aerobic, semiaerobic, and anaerobic photosynthetic ubiquinol oxidase gene expression patterns in the wild-type parent strain SB1003 and various regulatory mutants, as indicated below each bar. The values represent averages of at least three independent assays (standard deviations indicated by the error bars). Units of activity refer to the number of micromoles of o-nitrophenyl-ß-D-galactopyranoside hydrolyzed per minute per milligram of protein.
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In Escherichia coli, Fnr is one of several regulators of terminal oxidase gene expression (4). Since R. capsulatus contains a homolog of Fnr (27), we constructed a mutation in the chromosomal copy of fnrL and tested the mutant strain for its effect on terminal oxidase gene expression. The bar graph in Fig. 2 shows that there is no effect of disrupting fnrL on ubiquinol oxidase expression when the cells are grown strictly aerobically or anaerobically. In contrast, there is a reproducible 2.5- to 3-fold increase in ubiquinol oxidase expression from that in the wild type under semiaerobic growth conditions. This pattern has also been observed for cytochrome o oxidase expression in E. coli, where fnr mutations only show a significant effect under semiaerobic growth conditions (22).
In addition to the above tested "global regulators" that are found in many photosynthetic and nonphotosynthetic species, we also tested whether two aerobic repressors of the photosystem, CrtJ and AerR, are also involved in controlling ubiquinol oxidase gene expression. Analysis of ubiquinol oxidase expression indicates that aerR and crtJ mutants exhibit a twofold increase in expression aerobically and no effect anaerobically. This is very similar to the effect on bch, crt, and puc expression that is also observed upon disruption of these regulators (5, 8).
We also addressed the issue of dominance by constructing regA-fnrL and regA-crtJ double mutants. The ubiquinol oxidase expression pattern exhibited by the regA-crtJ double mutant was the same as that observed with the regA mutant under all three growth conditions. The regA mutant phenotype also prevailed in the regA-fnrL mutant when grown aerobically and semiaerobically. However, under anaerobic (photosynthetic) conditions, the regA-fnrL mutant showed a rather unexpected phenotype of no growth.
Cytochrome cbb3 oxidase expression. To assay the expression of cytochrome cbb3 oxidase, a translational lacZ fusion to the first gene in the ccoNOPQ operon was constructed (pDSccoN2) that contained 466 bp of DNA upstream of ccoN (20). This plasmid was mobilized into the same set of regulatory mutants and assayed in the same manner as described above for ubiquinol oxidase.
The expression pattern that we observed for cbb3 oxidase in wild-type R. capsulatus was also similar to that reported by Swem et al. (21) (Fig. 3). Specifically, expression was highest under semiaerobic conditions, intermediate under aerobic conditions, and lowest under anaerobic growth conditions.
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FIG. 3. ß-Galactosidase analysis of aerobic, semiaerobic, and anaerobic photosynthetic cytochrome cbb3 oxidase gene expression patterns in the wild-type parent strain SB1003 and various regulatory mutants as indicated below each bar. The values represent averages of at least three independent assays. Units of activity refer to the number of micromoles of o-nitrophenyl-ß-D-galactopyranoside hydrolyzed per minute per milligram of protein.
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FIG. 4. ß-Galactosidase analysis of aerobic, semiaerobic, and anaerobic photosynthetic cytochrome cbb3 oxidase gene expression gene expression patterns in the wild-type parent strain SB1003, as well as in regA- and regB-disrupted strains. The values represent averages of at least three independent assays. Units of activity refer to the number of micromoles of o-nitrophenyl-ß-D-galactopyranoside hydrolyzed per minute per milligram of protein.
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One of the significant effects on cbb3 oxidase expression occurs upon disruption of fnrL. In this case there is no effect on aerobically grown cells, a slight (30%) reduction under semiaerobically grown cells, and an 82% reduction in anaerobically grown cells. The effect of FnrL is even more obvious in semiaerobically grown cells when the fnrL mutation is present in conjunction with a regA mutation. In this case the effect is a 96% reduction in expression.
The crtJ and aerR regulatory mutants were also assayed for effects on ccoN::lacZ expression. Neither of these mutants exhibited a significant difference in expression from that found in the wild type under any growth conditions (data not shown).
The involvement of multiple regulators. There are two primary conclusions that can be derived from the results of this study. One conclusion is that R. capsulatus uses multiple regulators to ensure preferential and complete use of O2 as an electron acceptor by varying the level of cytochrome cbb3 oxidase and ubiquinol oxidase expression in response to changes in oxygen tension (Fig. 5). The second conclusion is that many transcription factors involved in controlling photosystem gene expression are involved in differentially controlling respiratory gene expression.
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FIG. 5. Regulatory scheme for control of the terminal oxidase operons in R. capsulatus. In response to oxygen availability, the regulators provide negative or positive transcriptional control to coordinate enzyme synthesis for optimal growth. The regulators provide control as follows: (i) RegB-RegA provides regulation in response to both aerobiosis and anaerobiosis, (ii) FnrL provides regulation in response to anaerobiosis, (iii) HvrA provides regulation in response to anaerobiosis, (iv) CrtJ provides regulation in response to aerobiosis, and (v) AerR provides regulation in response to aerobiosis. The operons code for the following genes: cydAB, ubiquinol oxidase; and ccoNOPQ, cytochrome cbb3 oxidase. Positive control (transcriptional activation) (+) and negative control (transcriptional repression) (-) of the genes are indicated.
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The actual affinities of the R. capsulatus cytochrome cbb3 and ubiquinol oxidases for oxygen have not yet been established. However, as is the case for E. coli (22), both of the R. capsulatus oxidases are expressed maximally under semiaerobic conditions. Also like E. coli, the aerobic and anaerobic expression patterns of the two R. capsulatus oxidases are reciprocal (Fig. 2 and 3). This pattern of expression suggests that cytochrome cbb3 is the low-affinity oxidase and that ubiquinol is the high-affinity oxidase. In order to maintain the reciprocal pattern of expression of the terminal oxidases, the cell must regulate transcription of each oxidase according to oxygen availability.
It has been demonstrated that the RegB-RegA regulatory system plays a key role in the transcriptional regulation of the terminal oxidases in R. capsulatus. In addition to RegA, R. capsulatus utilizes a homolog of E. coli Fnr to differentially control terminal oxidase gene expression. In E. coli Fnr functions as a repressor of cytochrome o oxidase semiaerobically but has no effect anaerobically. For cytochrome d oxidase, Fnr switches roles by functioning as an activator semiaerobically and as a repressor anaerobically (4). Similarly, the fnrL mutant in R. capsulatus also has no effect anaerobically on ubiquinol oxidase expression but acts as a repressor semiaerobically (Fig. 2). Conversely, FnrL acts as an activator of cbb3 oxidase expression both semiaerobically and anaerobically (Fig. 3). The phenotype of the regA-fnrL double mutant in R. capsulatus is intriguing though presently unexplainable. In addition to the substantially lowered terminal oxidase expression observed in these mutants, there is also the lack of growth under photosynthetic (anaerobic) conditions. The exact explanation for the nongrowth phenotype observed has yet to be uncovered, but it may be noted that an arcA-fnrL double mutant in E. coli also demonstrates insignificant expression of cytochrome d oxidase regardless of the availability of oxygen (9).
In addition to the regulators with functional similarities to E. coli oxidase regulators, there are other "photosystem specific" oxygen-responsive regulators utilized by R. capsulatus. Specifically, aerobic repression of ubiquinol oxidase appears to be promoted by the photopigment repressors CrtJ and AerR. These two repressors are known to be responsible for aerobic repression of bacteriochlorophyll, carotenoid, light harvesting, and reaction center gene expression (5, 8, 17). Our observation that CrtJ and AerR also aerobically repress expression of ubiquinol oxidase (Fig. 2) further supports the recent observation that R. capsulatus coordinates many different aerobic and anaerobic processes, such as photosynthesis (16), respiration (21), nitrogen fixation (7), carbon fixation (23), and hydrogen utilization (7). This appears to be mediated in part through the use of global response regulators such as RegB-RegA, as well as by CrtJ and AerR, which have overlapping photosynthesis and nonphotosynthesis target genes.
The utilization of multiple regulators for this process may be explained through their various sensitivities to oxygen that can be observed in this study (Fig. 5). As oxygen becomes depleted, some regulators may lose activity while others gain activity. For this reason it would be necessary for multiple regulators to be utilized so that the cell can optimally coordinate the synthesis of respiratory, photosynthetic, and biosynthetic processes according to the available levels of oxygen. The remaining challenge will be to determine mechanistic details of how the two terminal oxidase promoters are controlled by the aforementioned transcription factors and whether many of the observed effects observed are the direct result of interactions of these regulators with these respiratory promoters.
This work was supported by National Institutes of Health grants GM 53940 and GM40941.
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