Molecular Biology

Caught in the Act

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Science  01 Jun 2001:
Vol. 292, Issue 5522, pp. 1611
DOI: 10.1126/science.292.5522.1611d

The Balbiani ring 3 (BR3) gene of the dipteran Chironomus tentans contains 38 introns. These introns are roughly evenly spaced throughout the gene, and it is known that many are removed before synthesis of the RNA transcript is completed.

Taking advantage of the decondensed chromatin structure of active BR3 genes from the salivary gland, Wetterberg et al. have used electron tomographic reconstruction to visualize complexes of transcription and splicing factors at work. They found about 20 to 25 so-called NTS complexes on a BR3 gene, with immunolabeling evidence for the COOH-terminal domain of RNA polymerase II in the portion of the complex closer to the chromatin fiber and the U2 snRNP located within the distal regions of the complex. There were three main classes of NTS complex, with molecular mass ranging from 3.9 to 6.3 megadaltons. The approximate summed mass of the spliceosomal components and the lack of correlation between size and distance from one end of the gene together suggest that a transcribing polymerase recruits only one spliceosome and that splicing factors associate dynamically during the 15 to 25 seconds needed to excise an intron. — GJC

EMBO J.20, 2564 (2001).

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