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In the unicellular green alga Chlamydomonas reinhardtii there are two highly expressed derepressible phosphatases (DN and DA). Mutants altered in DN phosphatase (phoN) or/and DA phosphatase (phoA) activities are available. From the double mutant phoN6 phoA4 showing only some residual phosphatase activity at alkaline pH, a mutant (phoN6 phoA4 R18) displaying higher phosphatase activity at pH 9.5 was isolated. Comparative study of the phosphatase produced by the two strains revealed the enzymes to have very similar properties (Km for α-naphthylphosphate, thermosensitivity, pH dependence). In addition, a 58 kDa polypeptide present in the wild-type strain and previously described as a component of a derepressible phosphatase, was shown to be more abundant in phoN6 phoA4 R18. These results provide strong evidence that the residual enzyme in phoN6 phoA4 corresponds to a minor hitherto unknown phosphatase (DM) which is present in minute amounts in all Pi-starved cells (including wild-type where it is masked by the other phosphatases) and overproduced in phoN6 phoA4 R18. |
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