Constitutive heterochromatin in Microtus agrestis: binding of actinomycin-D and transcriptional inactivity.

Abstract

The constitutive heterochromatin of the field vole, Microtus agrestis, fails to incorporate 3H-uridine. It must be considered to be transcriptionally inactive. Nevertheless, in both living and fixed cells constitutive heterochromatin binds labelled actinomycin-D (3H-AMD) at the same rate or at even a higher rate than euchromatin per unit DNA. Extraction of histones by acids leads to an increase of the AMD-binding capacity of the entire chromatin. The number of free AMD-binding sites is found to be largely independent of the degree of chromatin condensation and heteropycnosis.-It is suggested that AMD-binding to constitutive heterochromatin reflects a genetic inactivity which is not necessarily connected with a repressive function of histones and with chromatin condensation. In this respect a distinct difference seems to exist between constitutive and facultative heterochromatin.