DNA methylation and RNA transcriptional activity in amphibian lampbrush chromosomes

Lampbrush chromosomes simultaneously exhibit two types of chromatin: the actively transcribed chromatin of lateral loops and the transcriptionally inactive chromatin of the chromosome axis. In situ localization of 5-methylcytosine in lampbrush chromosomes of the amphibian Pleurodeles waltlii, using specific antibodies to 5-methylcytosine, allowed us to demonstrate an inverse relationship between RNA transcriptional activity and the level of DNA methylation by light and electron microscopy using immunofluorescence and immunogold staining. The 5-methylcytosine was exclusively located in transcriptionally inactive chromatin in the chromosome axis and in the untranscribed spacers of some loops when the deoxyribonucleoprotein (DNP) axis was not wholly transcribed. In the vast majority of loops where the DNP axis was wholly transcribed, no methylated cytosine residues were ever detected. These results indicate a close relationship between the functional state of chromatin and the level of DNA methylation and suggest that DNA methylation is closely linked to gene control mechanisms in lampbrush chromosomes, i.e. at a definite time during oogenesis.