Pattern of binding of tritiated actinomycin D to Phaseolus coccineus polytene chromosomes. II. The entire chromosome complement

Distribution of 5-methylcytosine-rich regions in the polytene chromosomes of Phaseolus coccineus embryo suspensor as shown by the immunoperoxidase technique

5-Methylcytosine (5-mC) has been visualized in polytene chromosomes of Phaseolus coccineus, scarlet bean using specific antibodies to 5-mC and the immunoperoxidase technique. The results obtained indicate that most heterochromatic regions are methylated, even though the frequency of methylation is highly variable and sometimes low. A preferential binding of anti-5-mC to centromeric heterochromatic blocks was observed. Comparison between anti-5-mC binding and the results of hybridization with highly repetitive DNA and satellite DNA shows, moreover, that centrometric heterochromatic regions hybridize in particular with both DNAs. This finding is consistent with the fact that repetitive DNA and satellite DNA are methylated to a considerably greater extent than main band DNA, in line with many data to be found in the literature. The binding pattern of anti-5-mC that we observed also suggests that methylation does not occur in all classes of repetitive DNA. The high variability of band methylation frequency is discussed in relation to a possible characteristic DNA composition of the band.