ORNITHINE CYCLE IN NOSTOC PCC-73102 - PRESENCE OF AN IN-VITRO FUNCTIONAL ARGININOSUCCINATE LYASE

The presence of argininosuccinate lyase (ASL), an enzyme catalyzing th e final step of arginine biosynthesis, was demonstrated in the heteroc ystous cyanobacterium Nostoc PCC 73102 by measuring in vitro enzyme ac tivity and by visualization of ASL in native protein gels. Activity st aining of a native PAGE gel revealed one ASL-dependent band with a mol ecular mass of about 240 kDa. A colorimetric assay for ASL based on th e measurement of urea produced from arginine in the presence of an exc ess of arginase was further used to analyze the cyanobacterial ASL. Th e in vitro ASL activity was highest in cells in exponential growth pha se and decreased significantly during the stationary phase of growth, ranging from 4.4 to 0.8 nmol of product formed (mg protein)(-1) min(-1 ). Including arginine, citrulline or ornithine in the growth medium re sulted in no significant change in the ASL activities, indicating that Nostoc PCC 73102 ASL is not regulated by metabolites of the ornithine cycle.