The differential regulation of the synthesis of ribosomal RNA, 5 S RNA, and 4 S RNA in the polytenic salivary gland cells of the blowfly, Calliphora erythrocephala

Abstract

Third-instar larvae of the blowfly Calliphora erythrocephala were injected with [2- 3H]adenosine, and its flow into the salivary gland ATP pool and each of several electrophoretically resolved salivary gland RNA species were quantitated. From these data, the individual in vivo rates of synthesis, accumulation, and processing of salivary gland ribosomal RNA (rRNA), 4 S RNA, and 5 S RNA have been measured at several different developmental stages. These results indicate that the synthesis of 5 S RNA and rRNA are coordinate, developmentally regulated, and independent of the synthesis of 4 S RNA. A nonribosomal, heterodisperse RNA component (hdRNA) was also identified. This species contributes to both the rapidly turning over pulse-labeled RNA and the accumulating pulse-labeled RNA populations. Indirect measurements suggest that the developmental pattern of regulation of this RNA species is also independent of 5 S RNA and rRNA synthesis. The rate of synthesis and accumulation of each of these RNA species either remained constant or declined during the first three-fourths of the instar, despite a six- to sevenfold increase in the content of cellular DNA.