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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.