New polypeptides and in-vitro-translatable mRNAs are produced by phosphate-starved cells of the unicellular algaChlamydomonas reinhardtii

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Abstract
Cells of the unicellular algaChlamydomonas reinhardtii Dang. deprived of inorganic phosphate (Pi) secrete into the culture medium large amounts of glycoproteins which are not produced by cells grown in the presence of Pi. One of the polypeptides (P6: Mr 73000 ± 2 000) resolved by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) is absent from a mutant lacking neutral-phosphatase activity, and probably corresponds to a subunit of this enzyme. The antibodies raised to P6, however, were able to cross-react on Western blots with most of the secreted proteins which indicates that they recognize oligosaccharide epitopes common to all of these de-novo-formed molecules. In order to verify whether the response to Pi deprivation takes place at the transcriptional level, the in-vitro translation products directed by poly(A)+ RNA preparations obtained from cells grown with or without Pi were analyzed by two-dimensional SDS-PAGE. Three polypeptides specific to Pi-starved cells were detected but were considered unlikely to correspond to subunits of the neutral phosphatase. Whereas in-vivo-labelled proteins (notably P6) were precipitable by our antibodies, all attempts at precipitating in-vitro translation products have failed. This result is in agreement with the hypothesis that the antibodies recognize the oligosaccharide side chains but not the polypeptide backbone of the glycoproteins, a situation already described for monoclonal antibodies to the major structural glycoprotein of theChlamydomonas cell wall.