Transcription of tandemly repeated genes in embryonal and larval tissues of Calliphora erythrocephala

Abstract

The transcription of rRNA genes in Calliphora erythrocephala embryonal and larval fat bodies and guts was analyzed by electron microscopy. A gradual increase in the number of active genes and in the RNA polymerase density was observed in both tissues in the hours preceding hatching. Despite this increase, the rRNA genes at hatching are not fully loaded with RNA polymerase molecules. An additional increase in rRNA synthesis is characteristic of the first two days of larval development. This increase in rRNA synthesis is accomplished by an increase in RNA polymerase density, by activation of previously inactive rRNA genes, and by activation of newly replicated rRNA genes. The rRNA seems to be generated exclusively on genes not interrupted by a 6.1 kb intervening sequence. After the first 2 days, the RNA polymerase densities decline. Synthesis of rRNA seems to be coordinately regulated in these two tissues. — Two classes of tandemly repeated nonribosomal transcription units were detected in the fat bodies. The first class has an estimated DNA length of 3.3 kb and cannot be assigned to any of the known genes. The second class appears to represent very short transcription units with no visible RNP fibrils and is a candidate for 5S or tRNA transcription units.