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Molecular and Cellular Biology, December 1998, p. 7235-7242, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Endogenous Fluctuations of DNA Topology in the Chloroplast of Chlamydomonas reinhardtii

Maria L. Salvador,1 Uwe Klein,2 and Lawrence Bogorad3,*

Department of Biochemistry and Molecular Biology, University of Valencia, Burjassot, Valencia 46100, Spain1; Department of Biology, University of Oslo, Blindern, 0316 Oslo, Norway2; and The Biological Laboratories, Harvard University, Cambridge, Massachusetts 021383

Received 19 May 1998/Returned for modification 14 July 1998/Accepted 28 August 1998

DNA supercoiling in the chloroplast of the unicellular green alga Chlamydomonas 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.

* Corresponding author. Mailing address: The Biological Laboratories, Harvard University, 16 Divinity Ave., Cambridge, MA 02138. Phone: (617) 495-4292. Fax: (617) 495-4292. E-mail: bogorad{at}biosun.harvard.edu.

Molecular and Cellular Biology, December 1998, p. 7235-7242, Vol. 18, No. 12
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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