ROLE OF SILICON IN DIATOM METABOLISM - CYCLIC-NUCLEOTIDE LEVELS, NUCLEOTIDE CYCLASE, AND PHOSPHODIESTERASE ACTIVITIES DURING SYNCHRONIZED GROWTH OF CYLINDROTHECA-FUSIFORMIS

Adenylate cyclase, guanylate cyclase and the cyclic nucleotide phosphodiesterases of C. fusiformis were characterized in crude and partially purified preparations. Both cyclases were membrane-bound and required Mn2+ for activity, though Mg2+ gave 50% activity with adenylate cyclase. Properties of adenylate cyclase were similar to those of higher eukaryotic cyclases in some respects, and in other respects were like lower eukaryotic cyclases. Guanylate cyclase was typical of other lower eukaryotic enzymes. Two phosphodiesterase activities were found, one selective for cyclic AMP, the other for cyclic GMP. The 5''-nucleoside monophosphate was the major product of both activities and each of the enzymes had distinctive divalent cation requirements, pH optima and kinetic parameters. Both phosphodiesterases were similar to those of other lower eukaryotes with 1 notable difference: the cyclic AMP enzyme was inhibited by Ca. Changes in the cyclic nucleotide levels were quantitated in light-dark and Si-starvation synchronized cultures using a more sensitive radioimmunoassay than used in a previously published study (Borowitzka and Volcani 1977). Contrary to the previous report, the cyclic GMP level did not change significantly in either synchrony. The cyclic AMP level increased dramatically early in the period of DNA replication with the peak cyclic AMP accumulation substantially preceding that of DNA synthesis in both synchronies. There was no significant change in the activity of either cyclase or either phosphodiesterase during either synchrony. Thus, the mechanism for the rise in cAMP level remains unclear.