Ornithine cycle inNostoc PCC 73102: Stimulation of in vitro ornithine carbamoyl transferase activity by addition of arginine

Abstract

Cells ofNostoc PCC 73102, a free-living cyanobacterium originally isolated from the cycadMacrozamia, were cultured under different conditions and examined for the presence ofin vitro active ornithine carbamoyl transferase (OCT). Cells grown in darkness showed a significant increase ofin vitro OCT activity compared with the activity when grown in light. Addition of external arginine in the growth medium increasedin vitro OCT activity both in light and in darkness. Moreover, the highestin vitro OCT activity was observed in cells grown in darkness and with the addition of external arginine, a sevenfold increase compared with cells grown in light. Native-PAGE in combination with on gel OCT activity stain demonstrated that external arginine induced the presence of twoin vitro active OCT. In addition to the previously described 80 kDa OCT [Physiol Plant 84:275–282, 1992], a secondin vitro active enzyme with a molecular weight of approximately 118 kDa appeared. Western immunoblots, with native cell-free extracts and antibodies directed either against native or denatured OCT purified fromPisum sativum, confirmed that both enzymes were OCT. Moreover, with a denatured cell-free extract only one polypeptide, with a molecular weight of about 40 kDa, was recognized, indicating that the secondin vitro active OCT might be a trimer with three identical subunits.