Sensing of nitrogen limitation by Bacillus subtilis: Comparison to entericbacteria

Previous studies showed that Salmonella typhimurium apparently senses exter nal nitrogen limitation as a decrease in the concentration of the internal glutamine pool. To determine whether the inverse relationship observed betw een doubling time and the glutamine pool size in enteric bacteria was also seen in phylogenetically distant organisms, we studied this correlation in Bacillus 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 of B. subtilis clearly decreased under cer tain conditions of nitrogen limitation, particularly in continuous culture, the inverse relationship seen between glutamine pool size and doubling tim e in enteric bacteria was far less obvious in B. subtilis. To rule out the possibility that differences were due to the fact that B. subtilis has only a single pathway for ammonia assimilation, me disrupted the gene (gdh) tha t encodes the biosynthetic glutamate dehydrogenase in Salmonella, Studies o f the S. typhimurium gdh strain in ammonia-limited continuous culture and o f gdh glnA double mutant strains indicated that decreases in the glutamine pool remained profound in strains with a single pathway for ammonia assimil ation. Simple working hypotheses to account for the results with B. subtili s are that this organism refills an initially low glutamine pool by diminis hing the utilization of glutamine for biosynthetic reactions and/or repleni shes the pool by means of macromolecular degradation.