The repetition frequency of DNA in Balbiani ring 2 of Chironomus thummi

The RNA of Balbiani ring BR2 of polytene chromosomes from Chironomus thummi salivary glands was microisolated and reassociated in the presence of an excess of total larval DNA. BR2 RNA reacts as a single component with a C₀t_1/2 of 8.6. Ribosomal precursor RNA from microisolated nucleoli reassociates under identical conditions with a C₀t_1/2 of 12.3. These C₀t_1/2-values suggest repetition frequencies in the range of 35 and 50 for ribosomal DNA and Balbiani ring 2 DNA, respectively. The data presented here favour the view that the gene for BR2 RNA of C. thummi is internally repeated and contains only one type of DNA sequence.