BETA-HETEROCHROMATIN IN MAMMALS - EVIDENCE FROM STUDIES IN MICROTUS-AGRESTIS BASED ON THE EXTENSIVE ACCUMULATION OF L1 AND NON-L1 RETROPOSONS IN THE HETEROCHROMATIN

The heterochromatin of Microtus agrestis contains two retroposons, one L1 and one non-L1, which were cloned and analyzed with respect to the ir structure and genomic organization. These sequences have accumulate d in the heterochromatin and exhibit a complex interspersed organizati on of relatively recent origin. In contrast, the pericentromeric heter ochromatin is composed of simple, repetitive, tandemly organized eleme nts. From the underlying sequences, the dinucleotide frequencies, the sequence organization, and its transcriptional activity, the heterochr omatin of M. agrestis strongly resembles the beta-heterochromatin firs t described by Heitz and well characterized in Drosophila. This is evi dence that this class of heterochromatin, presumably with its own dist inct physiological and Functional role, is indeed also present in the mammalian genome.