The Role and Regulation of Quinol Oxidase in Rhizobium Etli

Aerobic bacteria typically respire by delivering electrons to oxygen via cytochrome c oxidases. In addition, many bacteria can respire by transferring electrons to oxygen via quinol oxidases. Exactly why bacteria contain quinol oxidases when they already have cytochrome c oxidases is unclear. In particular, the Cyo quinol oxidase, encoded by cyoABCD, is widespread among the dominant bacterial groups but its specific role is unknown. Rhizobium etli CFN42 provides an aerobic respiratory model in which Cyo can be directly compared to cytochrome c oxidases within the same organism. Mutants of the terminal oxidases in R. etli were constructed and their ability to grow in different physiological conditions was examined. The cyo mutant had noticable growth defects in low oxygen, low pH and low iron conditions. Furthermore, the cyo gene in the wild type was significantly up-regulated in these conditions. Conversely, in slow-growth conditions, such as stationary growth or growth in certain carbon sources, cyo was significantly down-regulated. A respiratory mutant, in which Cyo is the only viable terminal oxidase, had clear phenotypic defects in these conditions due to the regulation of cyo. Examination of the 5’ promoter region of cyo revealed multiple DNA binding sites for the transcription factor, ActR of the ActSR 2-component system. An actSR mutant was constructed and had significantly lower levels of cyo expression in all physiological conditions tested. The results suggest Cyo is important for growth and adaptation to low oxygen conditions, low pH, and low iron conditions in R. etli. Having an oxidase that enables the bacterium to aerobically respire in these conditions is of great benefit for bacteria, in particular soil bacteria that have to frequently adjust to adverse environmental conditions. Interestingly, each of these conditions would theoretically lead to higher quinol:quinone ratios in the cell. The activity of the transcriptional activator of cyo, ActSR, has been linked to the redox state of quinone in other bacteria, where quinone has been shown to inhibit ActSR activity. Altogether, the results indicate that the quinol:quinone ratio in the cell may be an important cue for Cyo utilization and expression

Quinol Oxidase Encoded by \u3cem\u3ecyoABCD\u3c/em\u3e in \u3cem\u3eRhizobium etli\u3c/em\u3e CFN42 is Regulated by ActSR and is Crucial for Growth at Low pH or Low Iron Conditions

Rhizobium etli aerobically respires with several terminal oxidases. The quinol oxidase (Cyo) encoded by cyoABCD is needed for efficient adaptation to low oxygen conditions and cyo transcription is upregulated at low oxygen. This study sought to determine how transcription of the cyo operon is regulated. The 5′ sequence upstream of cyo was analysed in silico and revealed putative binding sites for ActR of the ActSR two-component regulatory system. The expression of cyo was decreased in an actSR mutant regardless of the oxygen condition. As ActSR is known to be important for growth under low pH in another rhizobial species, the effect of growth medium pH on cyo expression was tested. As the pH of the media was incrementally decreased, cyo expression gradually increased in the WT, eventually reaching ∼10-fold higher levels at low pH (4.8) compared with neutral pH (7.0) conditions. This upregulation of cyo under decreasing pH conditions was eliminated in the actSR mutant. Both the actSR and cyo mutants had severe growth defects at low pH (4.8). Lastly, the actSR and cyo mutants had severe growth defects when grown in media treated with an iron chelator. Under these conditions, cyo was upregulated in the WT, whereas cyo was not induced in the actSR mutant. Altogether, the results indicated cyo expression is largely dependent on the ActSR two-component system. This study also demonstrated additional physiological roles for Cyo in R. etli CFN42, in which it is the preferred oxidase for growth under acidic and low iron conditions

A Quinol Oxidase, Encoded by \u3cem\u3ecyoABCD\u3c/em\u3e, Is Utilized to Adapt to Lower O\u3csub\u3e2\u3c/sub\u3e Concentrations in \u3cem\u3eRhizobium etli\u3c/em\u3e CFN42

Bacteria have branched aerobic respiratory chains that terminate at different terminal oxidases. These terminal oxidases have varying properties such as their affinity for oxygen, transcriptional regulation and proton pumping ability. The focus of this study was a quinol oxidase encoded by cyoABCD. Although this oxidase (Cyo) is widespread among bacteria, not much is known about its role in the cell, particularly in bacteria that contain both cytochrome c oxidases and quinol oxidases. Using Rhizobium etli CFN42 as a model organism, a cyo mutant was analysed for its ability to grow in batch cultures at high (21 % O2) and low (1 and 0.1 % O2) ambient oxygen concentrations. In comparison with other oxidase mutants, the cyo mutant had a significantly longer lag phase under low-oxygen conditions. Using a cyo :: lacZ transcriptional fusion, it was shown that cyo expression in the wild type peaks between 1 and 2.5 % O2. In addition, it was shown with quantitative reverse transcriptase PCR that cyoB is upregulated approximately fivefold in 1 % O2 compared with fully aerobic (21 % O2) conditions. Analysis of the cyo mutant during symbiosis with Phaseolous vulgaris indicated that Cyo is utilized during early development of the symbiosis. Although it is commonly thought that Cyo is utilized only at higher oxygen concentrations, the results from this study indicate that Cyo is important for adaptation to and sustained growth under low oxygen