URATE-MEDIATED REGULATION OF URATE OXIDASE IN CHLAMYDOMONAS-REINHARDTII

Expression of uricase (urate oxidase) from Chlamydomonas reinhardtii h as been investigated by using specific polyclonal antibodies. By Weste rn blot analyses performed under nondenaturing conditions, a 124 kDa p rotein band corresponding to active uricase was detected in protein ex tracts from cells cultured with urate or nitrogen-starved cells. This protein band was absent in cells cultured with ammonium. Besides the 1 24 kDa band, the antibodies also reacted with a massive protein band, with an apparent molecular mass of 500 kDa, that was detected in all n utritional conditions assayed. In vitro, inactive uricase from cells g rown with ammonium was activated by incubation in presence of urate. T he appearance of uricase activity in vitro coincided with a decrease o f the 500 kDa protein and an increase of the 124 kDa band correspondin g to the active enzyme. We suggest that a posttranslational regulation of uricase synthesis takes place in C. reinhardtii, and that urate ma y be responsible for the assembly or maturation of inactive precursors to form the active uricase.