Evaluation of the Sensitivity of the Terminal Deoxynucleotidyl TransferaseImmunogold Technique on Balbani Ring GenesMarc Thirya and Bertil Daneholtba Laboratory of Cell and Tissue Biology, University of Liège, Liège, Belgium b Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institute, Stockholm, Sweden Correspondence to: Marc Thiry, Laboratory of Cell and Tissue Biology, Univ. of Liège, Rue de Pitteurs, 20, B-4020 Liège, Belgium. Recently, we developed the terminal deoxynucleotidyl transferase (TdT)immunogold technique for in situ detection of DNA molecules. In this study the potential value and the limitations of the method were evaluated using the giant polytene chromosomes from Chironomus tentans salivary glands. Emphasis was put on the Balbiani rings (BRs), specialized chromosomal sites with exceptionally intense synthesis of large mRNA molecules. Immunolabeling was recorded not only over the bands and interbands of the polytene chromosomes but also over the BR structures. In the BRs, gold particles were present over segments of active transcription units, each with a central chromatin axis and a number of growing RNP products attached to the axis. One third of the transversely sectioned transcription units showed labeling in the central parts, i.e., where the unfolded chromatin axis is located, whereas the growing RNP fibers remained unlabeled. The absence of labeling of the RNP fibers is not likely to be due to lack of accessibility, because anti-RNA antibodies readily decorated the RNP fibers. The nuclear sap and cytoplasm displayed no significant label. These results clearly indicate that the TdTimmunogold technique is specific for DNA and detects not only DNA in compacted chromatin but also fully extended DNA. Its ability to efficiently label a single DNA molecule demonstrates the method's very high sensitivity. (J Histochem Cytochem 46:345351, 1998) Key Words: DNA, Balbiani ring genes, immunogold labeling, technique
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