SALMONELLA-TYPHIMURIUM APPARENTLY PERCEIVES EXTERNAL NITROGEN LIMITATION AS INTERNAL GLUTAMINE LIMITATION

Assimilation of nitrogen requires the synthesis of only two central in termediates, glutamate and glutamine, from which other compounds deriv e nitrogen by secondary transfers. We measured the internal Fool sizes of glutamate and glutamine in Salmonella typhimurium under conditions of external nitrogen limitation or sufficiency When growth was slowed by nitrogen limitation, the glutamine pool was lower by a factor of u p to 10, wheras the glutamate pool remained high. The decrease in the glutamine pool was general in nature, being seen with various limiting nitrogen sources in batch culture and with ammonia, the optimal nitro gen source, as the limiting nutrient in continuous culture. The only n itrogen source that gave discordant results was alanine, and we presen t evidence that alanine has inhibitory effects which extend beyond sim ple nitrogen limitation. Studies with mutant strains having altered ni trogen assimilation indicated that the decreases in the glutamine pool observed in the wild-type strain under nitrogen-limiting conditions w ere probably sufficient to account for slow growth and were likely to be responsible for slow growth. Hence we postulate that external nitro gen limitation is first perceived by Salmonella as a drop in its inter nal glutamine pool. (C) 1996 Academic Press Limited