RNP particles at splice junction sequences on Drosophila chorion transcripts.

HnRNP particles are located at specific sites on nascent transcripts when chromatin is spread for electron microscopic visualization. To determine if the sequences bound by the particles play a role in RNA processing, we have correlated the nascent transcript morphology of Drosophila chorion s36-1 and s38-1 genes with their nucleotide sequences. We find that RNP particles about 25 nm in diameter are at the splice junctions of the introns in these two transcripts. On the more mature chorion transcripts, a single larger (40 nm) particle is occasionally seen in the same vicinity, which probably results from the coalescence of the two smaller particles. This RNP structure may be involved in bringing splice junctions into close proximity and in maintaining this proximity during the bipartite splicing intermediate stage.