SUMMARY: During MgSO-induced modulation of , adenylate cyclase activity, histamine-sensitizing activity (HSA) and the major cell-envelope polypeptides with 28000 and 30000 (X polypeptides) were lost synchronously at a rate which could be accounted for by a simple growth-dilution effect. MgSO and other compounds which induced the above phenotypic change caused little inhibition of adenylate cyclase activity. Nicotinic acid was the sole exception and at 4·1 m-caused 60% inhibition of activity. Lysates of modulated cells, mixed with lysates of unmodulated cells, had no effect on either adenylate cyclase activity or HSA. Protein synthesis was a prerequisite for MgSO-induced modulation and also for the reversal of this process. Exogenous cAMP and dibutyryl cAMP (5 m) had no counteracting effect on MgSO- or nicotinic acid-induced modulation. The concentration of MgSO required to induce loss of the X polypeptides (10 to 11 m) was not altered by promoting adenylate cyclase activity by including an activator in the growth medium. In one culture containing 10 m-MgSO4 and activator, partial loss of the X polypeptides occurred and yet the extracellular cAMP concentration was twice that of cultures without activator and where full expression of the X polypeptides occurred. [H]cAMP-binding activity was detected in cell extracts of several strains of , but antiserum against purified catabote repressor protein gave no reaction with cell extracts. Respiration rates with amino acids were similar for modulated and unmodulated variants and an avirulent strain of These results are discussed in relation to a possible causal role for adenylate cyclase in modulation of


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