CAUTIONARY NOTE ON THE USE OF [METHYL-aH]THYMIDINE TO MEASURE RATES OF CHLOROPLAST DNA SYNTHESIS IN CHLAMYDOMONAS REINHARDTII

It has been reported that exogenously supplied thymidine and thymidine analogues, e.g. 5-bromodeoxyuridine, are specifically incorporated into chloroplast (B-component) D N A in Chlamydomonas reinhardtii (10). The chloroplast-specific incorporation is found in vegetative cells, but this specificity is lost during meiosis when both nuclear D N A and chloroplast D N A can be labeled by radioactive thymidine (1). The specificity in vegetative cells has been attributed (though not yet demonstrated) to the presence of thymidine kinase in the chloroplast and the absence of this enzyme in the nucleus and/or cytoplasm (1, 10). Since radioactive thymidine is incorporated only into chloroplast DNA, thymidine labeling, potentially, could be used to measure the rate of chloroplast D N A synthesis in whole cells. However, we have encountered problems in measuring rates of chloroplast D N A synthesis with the use of [methyl-3H]thymidine, due to a contaminant which is efficiently incorporated by C. reinhardtii cells. Several other reports in the literature warn about using [3H]thymidine to measure DNA synthesis in other organisms; however, these studies are concerned with the nature of the product into which bona fide thymidine is incorporated (2, 4, 6, 8).