Ultrastructure of a temperature-induced Balbiani ring in Chironomus thummi

A Balbiani ring-like structure (T-BR III) is induced at the right telomere on chromosome III by a 35 degrees C heat-shock. The location of T-BR III was identified in 3 micron-semithin sections that were afterwards resectioned to obtain ultrathin sections. These were stained either by uranyl acetate-lead citrate or PTA. The puff appeared composed of different structures: small compact chromatin bodies, loose chromatin with an apparently fibrillar organization, and granules. The granules, 200-250 A in diameter, appeared either in linear arrays or in a clustered form. The three components described above were interspersed within the T-BR without a compartmentalized organization. EDTA preferential ribonucleoprotein staining technique evidenced an EDTA-positive material within the T-BR that corresponded to 200-250 A granules as well as apparently fibrillar structures. However, EDTA did not completely stain some clustered granules. Neither free nor clustered granules were found in T-BRs formed in the presence of actinomycin D. The significance of the different T-BR structures in relation to the transcriptional activity of the puff is discussed.