Summary: Derepression of nitrogen fixation () genes in following transfer from NH -sufficiency to N-free medium was preceded rapid expansion of the guanosine 5′-di-phosphate 3′-diphosphate (ppGpp) pool. When derepressed in N-free medium supplemented with glutamine (600 μg ml), expression from the H and L promoters, determined as β-galactosidase activity in : : merodiploid strains, was stimulated 7-fold and nitrogenase activity 26-fold; ppGpp did not accumulate, remaining at the levels found in NH -repressed populations. The relaxed mutant , which accumulates only very low levels of ppGpp, showed partial derepression of nitrogenase activity in the presence of glutamine, thus ppGpp is unlikely to be an effector of expression. ATP and GTP levels were elevated under conditions where expression was enhanced, consistent with previous data suggesting that maintenance of ATP levels is a prerequisite for the expression of genes in


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