The Site of Synthesis of Two Chloroplast Cytochromes in Chlamydomonas reinhardi

The analysis of steps of genetic transcription and translation specifying certain of the chloroplast components of the unicellular green alga, Chlamydomonas reinhardi, has been the subject of several publications from this laboratory (1, 4, 7-10, 13, 16, 17). The cytochromes are of particular interest in this regard, for the chloroplasts of C. reinhardi contain at least three -cytochrome 553, cytochrome 559, and cytochrome 563 (5, 6, 12)-and there is some evidence to suggest (6, and B. Epel, unpublished) that there may be more than one cytochrome 559. The synthesis of cytochromes 553 and 563 has been shown to depend on translational steps that occur on chloroplast ribosomes, for their synthesis in synchronous cultures is prevented by antibiotics that inhibit translation on chloroplast ribosomes (1). An earlier paper from this laboratory reported (13) that the level of these two cytochromes was not affected in cells of a mutant strain of C. reinhardi in which the chloroplasts contain only 5 to 10% of the number of chloroplast ribosomes found in wild-type cells (9). The discrepancy between the results obtained with synchronous cultures of wildtype cells and chloroplast ribosome-deficient mutant strain, ac-20, has prompted the reinvestigation of the cytochrome content of the mutant strain, particularly in view of the fact that the determination of the cytochromes reported in the earlier work was based on light-induced absorbance changes in chloroplast fragments, a less sensitive method of determination than one based on measurements of the chemically reduced-minusoxidized difference spectra of solubilized cytochromes 553 and 563. In this paper we show that both cytochrome 553 and 563 are deficient in mixotrophically grown cells of ac-20 in which there is a drastic reduction in the number of chloroplast ribosomes. We also show that the cytochrome content increased when cells of the mutant strain are placed under phototrophic growth conditions where the level of chloroplast ribosomes increases to at least 25% of the level characteristic of the wildtype strain (9). Thus, the results are in accord with the observation (1) that chloroplast ribosomes play an essential role in the synthesis of cytochromes 553 and 563.