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Nucleic Acids Research, 1984, Vol. 12, No. 3 1501-1516
© 1984

MOLECULAR BIOLOGY

5-Methyldeoxycytidine in the Physarum minichromosome containing the ribosomal RNA genes

Craig A. Cooney*, Harry R. Matthews and E.Morton Bradbury

Department of Biological Chemistry, School of Medicine, University of California Davis, CA 95616, USA

*To whom correspondence should be sent

Received August 18, 1983. Revised December 12, 1983. Accepted December 12, 1983.

5-Methyldeoxycytidine (5MC) was analyzed by high pressure liquid chromatography (HPLC) and by restriction enzyme digestion in rDNA isolated from physarum polycephalum rDNA from Physarum M3C strain microplasinodia has a significant 5MG content (about half that of the whole genomic DNA). This rDNA contains many C5MCGG sites because it is clearly digested further by Msp I than by Hpa II. However, most 5MG is in other sites. In particular, alternating CG sequences appear to be highly methylated. HPLC of deoxyrlbonucleo- sides shows that most of the transcribed regions contain little or no 5MC. Restriction digestion indicates that there is little or no 5MC In any of the transcribed regions including the transcription origin and adjacent sequences. Over 90% of the total 5MC is in or near the central nontranscribed spacer and most methylated restriction sites are in inverted repeats of this spacer. rDNA is very heterogeneous with respect to 5MC. The 5MC pattern doesn't appear to change with inactivation of the rRNA genes during reversible differentiation from microplasmodia (growing) to microsclerotia (dormant), showing that inactivation is due to changes in other chromatin variables. The 5MG pattern is different between Physarum strains. The possible involvement of this 5MC in rDNA chromatin structure and in cruciform and Z-DNA formation is discussed.


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