Characterization of the 32,000 Dalton Chloroplast Membrane Protein: III. Probing Its Biological Function in Spirodela

The rapidly turning over, photoinduced thylakoid protein, P-32000, is the main pulse-labeled membrane polypeptide in the chloroplasts of Spirodela oligorrhiza, yet little is known of its physiological function. Two hypotheses are tested: that P-32000 synthesis is necessary for thylakoid biogenesis; that it directly participates in photosynthesis. Spirodela cultures were dissected into expanding and fully mature tissue. Fronds from both developmental stages transcribed a 0.5 x 10(6) dalton RNA likely to be the message for P-32000. As to the protein itself, synthesis occurred in both types of tissue but was considerably enhanced in the fully mature state. Thus, a purely transient, developmental function for P-32000 during thylakoid biogenesis appears ruled out. Low concentrations of d-threo-chloramphenicol severely suppressed P-32000 synthesis but not its turnover. As a result, fronds depleted in P-32000 were obtained. However, photoassimilation of CO(2) remained at 86% of normal in tissue > 80% depleted of P-32000. Thus, P-32000 did not appear to be rate-limiting, suggesting that it does not serve as a direct, integral part of the photosynthetic pathway.