SUMMARY: strain 13189 produced a large number of submerged spores in response to nitrogen-mediated nutritional shift-down. The pool size of ppGpp increased immediately after shift-down and was accompanied by a dramatic decrease in the GTP pool. In contrast, nutritional shift-down of a relaxed () mutant derived from strain 13189 resulted in a slight increase in ppGpp pool size, a less extensive decrease in GTP pool size and spore titres 10-fold less than produced by the parental strain. Addition of decoyinine, a specific inhibitor of GMP synthetase, to the mutant restored the sporulation frequency to the parental level. IMP dehydrogenase was competitively inhibited by ppGpp with a Ki of 0-05 mM, whereas other nucleotides were less effective inhibitors. The observed intracellular concentration of ppGpp during shift-down was sufficient to inhibit IMP dehydrogenase activity and to result in the decrease in the GTP pool. These results demonstrate that sporulation of , like that of , is nutrient-dependent and that the decrease in GTP content caused by the stringent response (ppGpp) is correlated with initiation of submerged spore formation due to amino acid starvation.


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