Date of Award
Spring 2015
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Biological Sciences
First Advisor
Noel, Dale K.
Second Advisor
Hristova, Krassimira
Third Advisor
Stuart, Rosemary
Abstract
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 [sic] 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.