Mutants impaired in derepressible alkaline phosphatase activity in Chlamydomonas reinhardtii

In the green alga Chlamydomonas reinhardtii, inorganic phosphate starvation results in the synthesis and secretion of two classes of derepressible phosphatases, DN (pH optimum about 7.0) and DA (pH optimum 9.5). Mutants specifically impaired in DA phosphatase activity were isolated after UV treatment of the phoN6 strain lacking the DN phosphatase. Four mutants (phoA3, phoA4, phoA5, phoA6) were genetically analyzed. Mutations were allelic, non-complementing and recessive to the wild-type allele. The lack of DA phosphatase activity in phoA mutants was correlated with the absence of a high molecular weight (200 kDa) polypeptide (P2) present in the secretions of phoA(+) strains. In addition, partial purification of the DA phosphatase by anion-exchange chromatography in the presence of urea resulted in a concomitant enrichment in this polypeptide. These results indicate that PZ is likely a component of the DA phosphatase. Since the only detectable change in all phoA mutants was the disappearance of P2, it is proposed that phoA is a regulatory gene specifically involved in the control of the DA phosphatase.