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ABSTRACT
There was a long-held belief that the gram-positive soil bacterium Bacillus subtilis is a strict aerobe:. But recent studies have shown that B. subtilis will grow anaerobically, either by using nitrate or nitrite as a terminal electron acceptor, or by fermentation. How B. subtilis alters its metabolic activity according to the availability of oxygen and alternative electron acceptors is but one focus of study. A two-component signal transduction system composed of a sensor kinase, ResE, and a response regulator, ResD, occupies an early stage in the regulatory pathway governing anaerobic respiration. One of the essential roles of ResD and ResE in anaerobic gene regulation is induction of fnr transcription upon oxygen limitation. FNR is a transcriptional activator for anaerobically induced genes, including those for respiratory nitrate reductase, narGHJl. B. subtilis has two distinct nitrate reductases, one for the assimilation of nitrate nitrogen and the other for nitrate respiration. In contrast, one nitrite reductase functions both in nitrite nitrogen assimilation and nitrite respiration. Unlike many anaerobes, which use pyruvate formate lyase, B. subtilis can carry out fermentation in the absence of external electron acceptors wherein pyruvate dehydrogenase is utilized to metabolize pyruvate.
KEY WORDS: anaerobiosis, nitrate respiration, fermentation, gene regulation
INTRODUCTION
Bacteria often encounter drastic changes in their environment, including fluctuations in the levels of external oxygen. Unlike strict aerobes or strict anaerobes, which can survive only either in the presence or absence of oxygen, facultative anaerobes can cope with changes in environmental oxygen levels by sensing oxygen concentration and shifting cellular metabolism accordingly (28, 36, 37, 45, 76). The changes in metabolism in response to changes in oxygen availability include adjustments to the rate and route of carbon source utilization, to the pathways of electron flow to maintain an oxidation-reduction balance, and to the mechanisms of energy production and of certain biosynthetic reactions. These changes are achieved by modulating protein activity, by regulating the expression of the appropriate genes, or both. As one approach to studying these differences in the pattern of gene expression, proteins whose concentrations fluctuate in response to changes in oxygen levels have been identified by two-dimensional gel electrophoresis of extracts obtained from aerobically and anaerobically grown cultures of Escherichia coli (65, 71,72) and Salmonella typhimurium (1). Another...