INTRODUCTION OF A DNA METHYLTRANSFERASE INTO DROSOPHILA TO PROBE CHROMATIN STRUCTURE IN-VIVO

The dam DNA methyltransferase gene from Escherichia coli was introduce d into Drosophila in order to probe chromatin structure in vivo. Expre ssion of the gene caused no visible defects or developmental delay eve n at high levels of active methylase. About half of each target site w as found to be methylated in vivo, apparently reflecting a general pro perty of chromatin packaged in nucleosomes. Although site-specific dif ferences were detected, most euchromatic and heterochromatic sites sho wed comparable degrees of methylation, at least at high methylase leve ls. Methylase accessibility of a lacZ reporter gene subject to positio n-effect variegation throughout development was only slightly reduced, consistent with studies of chromatin accessibility in vitro. Silencin g of lacZ during development differed from silencing of an adjacent wh ite eye pigment reporter gene in the adult, as though chromatin struct ure can undergo dynamic alterations during development.