Two transcripts of an individual Balbiani ring from the salivary gland cells of Acricotopus lucidus (Diptera, Chironomidae).

The polytene chromosomes in main lobe cells of the Acricotopus salivary gland carry two large Balbiani rings (BR1, BR2), which BR1 exhibits the higher incorporation rate of tritiated nucleosides. The size of BR1 varies in conjunction with a subterminal inversion which includes the BR1 site. BR1 in both the homozygous and heterozygous inverted arrangement (BR1A and BR1B, respectively) is consistently smaller than in the homozygous standard arrangement (BR1C). The RNA content of BR1A and BR1C (3.1 and 6.7 pg, respectively) corresponds well with their relative size.--Using the method of micromanipulation of squashed salivary gland chromosomes the RNA transcripts of BR1A and BR1C have been extracted from microdissected Balbiani rings and fractionated by electrophoresis on 1% agarose gels. In BR1A one RNA fraction with a molecular weight of about 40 x 10(6) D is transcribed. The gel electrophoretic pattern of BR1C is characterized by an additional RNA fraction of 5 x 10(6) D. The main part of BR1 RNA is bound to oligo(dT)-cellulose suggesting the adenylation at the chromosome of Balbiani ring RNA.--In the profile of poly(A)-containing cytoplasmic RNA obtained after in vivo labelling two peaks can be detected which correspond in size to the BR1 RNA. These putative BR1 transcripts accumulate to a high concentration in the cytoplasm and show a relatively low stability during in vivo chase experiments.