ENDOGENOUS FLUCTUATIONS OF DNA TOPOLOGY IN THE CHLOROPLAST OF CHLAMYDOMONAS-REINHARDTII

DNA supercoiling in the chloroplast of the unicellular green alga Chla mydomonas reinhardtii was found to change with a diurnal rhythm in cel ls growing in alternating 12-h dark-12-h light periods. Highest and lo west DNA superhelicities occurred at the beginning and towards the end of the 12-h light periods, respectively. The fluctuations in DNA supe rcoiling occurred concurrently and in the same direction in two separa te parts of the chloroplast genome, one containing the genes psaB, rbc L, and atpA and the other containing the atpB gene. Fluctuations were not confined to transcribed DNA regions, indicating simultaneous chang es in DNA conformation all over the chloroplast genome. Because the di urnal fluctuations persisted in cells kept in continuous light, DNA su percoiling is judged to be under endogenous control. The endogenous fl uctuations in chloroplast DNA topology correlated tightly with the end ogenous fluctuations of overall chloroplast gene transcription and wit h those of the pool sizes of most chloroplast transcripts analyzed. Th is result suggests that DNA superhelical changes have a role in the re gulation of chloroplast gene expression in Chlamydomonas.