Sensing of Nitrogen Limitation by Bacillus subtilis: Comparison to Enteric Bacteria

Abstract

ABSTRACTPrevious studies showed thatSalmonella typhimuriumapparently senses external nitrogen limitation as a decrease in the concentration of the internal glutamine pool. To determine whether the inverse relationship observed between doubling time and the glutamine pool size in enteric bacteria was also seen in phylogenetically distant organisms, we studied this correlation inBacillus subtilis, a gram-positive, sporulating bacterium. We measured the sizes of the glutamine and glutamate pools for cells grown in batch culture on different nitrogen sources that yielded a range of doubling times, for cells grown in ammonia-limited continuous culture, and for mutant strains (glnA) in which the catalytic activity of glutamine synthetase was lowered. Although the glutamine pool size ofB. subtilisclearly decreased under certain conditions of nitrogen limitation, particularly in continuous culture, the inverse relationship seen between glutamine pool size and doubling time in enteric bacteria was far less obvious inB. subtilis. To rule out the possibility that differences were due to the fact thatB. subtilishas only a single pathway for ammonia assimilation, we disrupted the gene (gdh) that encodes the biosynthetic glutamate dehydrogenase inSalmonella. Studies of theS. typhimurium gdhstrain in ammonia-limited continuous culture and ofgdh glnAdouble-mutant strains indicated that decreases in the glutamine pool remained profound in strains with a single pathway for ammonia assimilation. Simple working hypotheses to account for the results withB. subtilisare that this organism refills an initially low glutamine pool by diminishing the utilization of glutamine for biosynthetic reactions and/or replenishes the pool by means of macromolecular degradation.