Endogenous Fluctuations of DNA Topology in the Chloroplast of Chlamydomonas reinhardtii

ABSTRACT

DNA supercoiling in the chloroplast of the unicellular green algaChlamydomonas reinhardtii was found to change with a diurnal rhythm in cells growing in alternating 12-h dark–12-h light periods. Highest and lowest DNA superhelicities occurred at the beginning and towards the end of the 12-h light periods, respectively. The fluctuations in DNA supercoiling occurred concurrently and in the same direction in two separate parts of the chloroplast genome, one containing the genes psaB, rbcL, and atpA and the other containing the atpB gene. Fluctuations were not confined to transcribed DNA regions, indicating simultaneous changes in DNA conformation all over the chloroplast genome. Because the diurnal fluctuations persisted in cells kept in continuous light, DNA supercoiling is judged to be under endogenous control. The endogenous fluctuations in chloroplast DNA topology correlated tightly with the endogenous fluctuations of overall chloroplast gene transcription and with those of the pool sizes of most chloroplast transcripts analyzed. This result suggests that DNA superhelical changes have a role in the regulation of chloroplast gene expression in Chlamydomonas.

ACKNOWLEDGMENTS

M. L. Salvador and U. Klein contributed equally to this work.

We thank Klaus Valentin, University of Gießen, Germany, for the gift of the plasmid containing the rbcL/S gene of E. siliculosus and Toril Håkestad, University of Oslo, Norway, for plasmid psaB:GUS.

The work was supported by grants from DGICYT (PB 95-1075) to M.L.S., the Norwegian Research Council (100946/410) to U.K., and the National Institute of General Medical Sciences of the N.I.H., U.S.P.S., to L.B.