The chromosomal distribution of balbiani ring dna in chironomus tentans

RNA from Balbiani ring 2 (BR 2) of Chironomus tentans salivary gland cells was hybridized to polytene chromosomes of rectum cells, in which the Balbiani ring is not expressed. The distribution of the RNA:DNA hybrids was studied by autoradiography, using ultrathin emulsion to increase the resolution. The grains were estimated to cover 3-5 polytene chromosome bands, which indicates that information expressed by transcription in BR 2 may be present in several adjoining chromomeres. From a comparison of grain numbers over BR 2 of salivary gland chromosomes and over the BR 2 region of rectum chromosomes, it appears unlikely that selective amplification of BR DNA in the salivary glands could explain the efficient hybridization seen in this tissue. The conclusion is advanced that BR 2 DNA contains nucleotide sequences which are extensively repeated within 3-5 transcription units.