A SPONTANEOUS CHANGE IN THE INTRACELLULAR CYCLIC-AMP LEVEL IN ASPERGILLUS-NIGER IS INFLUENCED BY THE SUCROSE CONCENTRATION IN THE MEDIUM AND BY LIGHT

A spontaneous rise in intracellular cyclic AMP (cAMP) levels was obser ved in the early stages of Aspergillus niger growth under conditions y ielding large amounts of citric acid, The amount of cAMP formed was fo und to depend on the initial concentration of sucrose in the medium, U nder higher-sucrose conditions, the cAMP peak appeared earlier and,vas higher, while in lower-sucrose media a flattened peak was observed la ter in fermentation, Since in media with higher concentrations of sucr ose intracellular citric acid starts to accumulate earlier and more ra pidly, cAMP synthesis may be triggered by intracellular acidification, which is caused by the dissociation of citric acid. No spontaneous in crease in cAMP concentrations could be detected when the cells were gr owth in continuously illuminated cultures, suggesting that A. niger ph osphodiesterase (PDE) is photoregulated, More evidence for the activat ion of PDE by light was obtained from morphological studies under ligh t and dark conditions in the presence of cAMP or N-6,O-2'-dibutyryl cA MP, and this idea was additionally supported by experiments in which P DE inhibitors were tested.