THE DYNAMICS OF A PRE-MESSENGER-RNA SPLICING FACTOR IN LIVING CELLS

Pre-mRNA splicing is a predominantly co-transcriptional event which in volves a large number of essential splicing factors(1,2). Within the m ammalian cell nucleus, most splicing factors are concentrated in 20-40 distinct domains called speckles(3). The function of speckles and the organization of cellular transcription and pre-mRNA splicing in vivo are not well understood. We have investigated the dynamic properties o f splicing factors in nuclei of living cells. Here we show that speckl es are highly dynamic structures that respond specifically to activati on of nearby genes. These dynamic events are dependent on RNA polymera se II transcription, and are sensitive to inhibitors of protein kinase s and Ser/Thr phosphatases. When single genes are transcriptionally ac tivated in living cells, splicing factors leave speckles in peripheral extensions and accumulate at the new sites of transcription, We concl ude that one function of speckles is to supply splicing factors to act ive genes. Our observations demonstrate that the interphase nucleus is far more dynamic in nature than previously assumed.