ORNITHINE CYCLE IN NOSTOC PCC-73102 - ARGINASE, OCT AND ARGININE DEIMINASE, AND THE EFFECTS OF ADDITION OF EXTERNAL ARGININE, ORNITHINE, ORCITRULLINE

Arginase, ornithine carbamoyl transferase (OCT) and arginine deiminase activities were found in cell-free extracts of Nostoc PCC 73102, a fr ee-living cyanobacterium originally isolated from the cycad Macrozamia . Addition of either arginine, ornithine or citrulline to the growth m edium induced significant changes in their in vitro activities. Moreov er, growth in darkness, compared to in light, induced higher in vitro activities. The in vitro activities of arginase and arginine deiminase , two catabolic enzymes primarily involved in the breakdown of arginin e. increased substantially by a combination of growth in darkness and addition of either arginine, or ornithine, to the growth medium. The m ost significant effects on the in vitro OCT activities where observed in cells grown with the addition of ornithine. Cells grown in darkness exhibited about 6% of the in vivo nitrogenase activity observed in ce lls grown in light. However, addition of external carbon (glucose and fructose) to cells grown in darkness resulted in in vivo nitrogenase a ctivity levels similar to, or even higher than, cells grown in light. Growth with high in vivo nitrogenase activity or in darkness with the addition of external carbon, resulted in repressed levels of in vitro arginase and arginine deiminase activities. It is suggested that nitro gen starvation induces a mobilization of the stored nitrogen, internal release of the amino compound arginine, and an induction of two catab olic enzymes arginase and arginine deiminase. A similar and even more pronunced induction can be observed by addition of external arginine t o the growth medium