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SUMMARY: Streptomyces griseus 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 (rel) 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 re1 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 S. griseus, like that of Bacillus subtilis, 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.