The distribution of nuclear ribonucleoprotein (hnRNP) particles in Drosophila polytene chromosomes has been investigated using anti-B-36 serum as a probe. The use of polytene chromosomes allows resolution at the level of the chromomere, and provides the opportunity to look for both positive and negative correlations with transcriptional activity. The antiserum was obtained using the nuclear protein B-36 from Physarum polycephalum as the immunogen. It has been shown to precipitate hnRNP particles from HeLa cells through a cross-reaction with the major 32,000- and 34,000-dalton hnRNP particle proteins. The antiserum cross- reacts with a Drosophila nuclear protein of approximately 34,000 daltons. By indirect immunofluorescence, we observed that the antiserum reacts preferentially with transcriptionally active loci of the polytene chromosomes, whereas loci previously or subsequently active do not show significant fluorescence. The overall pattern of fluorescence is very similar to that generated with anti-RNA polymerase B serum. The correlation of fluorescence and transcriptional activity observed suggests that the anti-B-36 serum is recognizing hnRNP proteins which have combined with nascent RNA molecules at the sites of transcription
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