Fermentative Metabolism of Bacillus subtilis : Physiology and Regulation of Gene Expression

Abstract

Bacillus subtilis grows in the absence of oxygen using nitrate ammonification and various fermentation processes. Lactate, acetate, and 2,3-butanediol were identified in the growth medium as the major anaerobic fermentation products by using high-performance liquid chromatography. Lactate formation was found to be dependent on the lctEP locus, encoding lactate dehydrogenase and a putative lactate permease. Mutation of lctE results in drastically reduced anaerobic growth independent of the presence of alternative electron acceptors, indicating the importance of NADH reoxidation by lactate dehydrogenase for the overall anaerobic energy metabolism. Anaerobic formation of 2,3-butanediol via acetoin involves acetolactate synthase and decarboxylase encoded by the alsSD operon. Mutation ofalsSD has no significant effect on anaerobic growth. Anaerobic acetate synthesis from acetyl coenzyme A requires phosphotransacetylase encoded by pta. Similar to the case for lctEP, mutation of pta significantly reduces anaerobic fermentative and respiratory growth. The expression of both lctEP and alsSD is strongly induced under anaerobic conditions. Anaerobic lctEP andalsSD induction was found to be partially dependent on the gene encoding the redox regulator Fnr. The observed fnrdependence might be the result of Fnr-induced arfM(ywiD) transcription and subsequent lctEP andalsSD activation by the regulator ArfM (YwiD). The two-component regulatory system encoded by resDE is also involved in anaerobic lctEP induction. No directresDE influence on the redox regulation ofalsSD was observed. The alternative electron acceptor nitrate represses anaerobic lctEP andalsSD transcription. Nitrate repression requiresresDE- and fnr-dependent expression ofnarGHJI, encoding respiratory nitrate reductase. The genealsR, encoding a regulator potentially responding to changes of the intracellular pH and to acetate, is essential for anaerobic lctEP and alsSD expression. In agreement with its known aerobic function, no obvious oxygen- or nitrate-dependent pta regulation was observed. A model for the regulation of the anaerobic fermentation genes in B. subtilis is proposed.