5-methylcytosine in the DNA of the polytene chromosomes of the diptera Sciara coprophila, Drosophila melanogaster and D. persimilis.

5-Methylcytosine has been detected in the DNA of the polytene chromosomes of Sciara coprophila, Drosophila melanogaster and D. persimilis, using specifically purified antibodies to 5-methylcytidine. The 5-methylcytosine is present in GC-rich sequences in the bands. Virtually no 5-methylcytosine is detectable in mitotic metaphase chromosomes of S. coprophila. Thus, methylation is associated with polytenization in these diptera. Restriction enzyme studies using HpaII, MspI, HhaI and AluI indicate that unmethylated 5'-CCGG-3', 5'-GCGC-3' and 5'-AGCT-3' sequences are abundant in polytene chromosome DNA. These sequences are probably not major sites of methylation. Since the DNA in the bands of polytene chromosomes is generally transcriptionally inactive, as well as extensively methylated, these results are consistent with the hypothesis that genes in the bands have been inactivated by a process involving DNA methylation.