Fermentative metabolism of Bacillus subtilis: Physiology and regulation ofgene expression

Bacillus subtilis grows in the absence of oxygen using nitrate ammonificati on and various fermentation processes. Lactate, acetate, and 2,3-butanediol were identified in the growth medium as the major anaerobic fermentation p roducts by using high-performance liquid chromatography. Lactate formation was found to be dependent on the lctEP locus, encoding lactate dehydrogenas e and a putative lactate permease. Mutation of lctE results in drastically reduced anaerobic growth independent of the presence of alternative electro n accepters, indicating the importance of NADH reoxidation by lactate dehyd rogenase for the overall anaerobic energy metabolism. Anaerobic formation o f 2,3-butanediol via acetoin involves acetolactate synthase and decarboxyla se encoded by the alsSD operon. Mutation of alsSD has no significant effect on anaerobic growth. Anaerobic acetate synthesis from acetyl coenzyme A re quires phosphotransacetylase encoded by pta. Similar to the case for lctEP, mutation of pta significantly reduces anaerobic fermentative and respirato ry growth. The expression of both lctEP and alsSD is strongly induced under anaerobic conditions. Anaerobic IctEP and alsSD induction was found to be partially dependent on the gene encoding the redox regulator Fnr. The obser ved fnr dependence might be the result of Fnr-induced arFM (ywiD) transcrip tion and subsequent IctEP and alsSD activation by the regulator ArfM (YwiD) . The two-component regulatory system encoded by resDE is also involved in anaerobic lctEP induction. No direct resDE influence on the redox regulatio n of alsSD was observed. The alternative electron acceptor nitrate represse s anaerobic IctEP and alsSD transcription. Nitrate repression requires resD E- and fnr-dependent expression of narGHJI, encoding respiratory nitrate re ductase. The gene alsR, encoding a regulator potentially responding to chan ges of the intracellular pH and to acetate, is essential for anaerobic IctE P and alsSD expression. In agreement with its known aerobic function, no ob vious oxygen- or nitrate-dependent pta regulation was observed. A model for the regulation of the anaerobic fermentation genes in B. subtilis is propo sed.