Differential representation of chromosomal HRNA in nuclear sap

Abstract

Heterogeneous, high molecular weight RNA (HRNA) has been characterized in different nuclear components of salivary gland cells in Chironomus tentans. HRNA synthesized in different chromosomal regions (chromosomes I–III and in four defined segments of chromosome IV) were compared with each other and with HRNA in the nuclear sap. HRNA from all the components studied consisted of molecules in a range between 10–15S and 80–90S. HRNA from chromosomes I–III and from three of the four chromosome IV segments had a similar base ratio ( CMP UMP activity quotient) and were not significantly different from each other. However, the segment on chromosome IV, containing Balbiani ring 2 (BR2), had a characteristic HRNA base ratio different from that of the other chromosomal regions. The base ratio of HRNA in nuclear sap was significantly different from that in the whole chromosome set but showed close agreement with that in BR2. HRNA in nuclear sap is consequently not represented there in proportion to its presence on the whole chromosome set but is instead heavily enriched in a type of HRNA probably synthesized in BR2. Although alternative explanations have to be considered, the favoured interpretation is that a selective degradation of HRNA takes place on the chromosomes (or shortly after a release of HRNA from the chromosomes), as most HRNA is rapidly degraded in the salivary gland cells, as in other nucleated cells.