EM autoradiographic studies on polytene nuclei of Drosophila melanogaster: II. Organization and transcriptive activity of the chromocentre

Abstract

The organization and the transcriptive activity of the chromocentre region of salivary gland nuclei of Drosophila melanogaster have been examined by electron microscopy and EM autoradiography. The heterochromatic chromocentre region is organized into a central compact block of chromatin, surrounded by a large number of smaller, interconnected blocks. The latter are continued into the euchromatic chromosome arms. The two types of chromatin organization in the chromocentre are reminiscent of the classical α- and β-heterochromatin. Two types of ribonucleoprotein-like particles regularly occur in the β-heterochromatin. EM autoradiography after short (5 min) in vitro pulse of ³H-uridine shows that the β-heterochromatin is as active in RNA synthesis as the euchromatin in the nucleus; the α-heterochromatin is completely inactive. It is suggested that the two types of particles seen in the β-heterochromatin are products of RNA synthesis occurring in this region. These observations raise doubts on the general validity of the notion of total inactivity of constitutive heterochromatin.