THE INTRANUCLEAR SITE OF EXCISION OF EACH INTRON IN BALBIANI RING-3 PRE-MESSENGER-RNA IS INFLUENCED BY THE TIME REMAINING TO TRANSCRIPTION TERMINATION AND DIFFERENT EXCISION EFFICIENCIES FOR THE VARIOUS INTRONS

The 10.9-kb Balbiani ring 3 (BR3) gene contains 38 constitutively exci sed introns. Both nascent and nucleoplasmic, released BR3 gene pre-mRN A can be isolated by microdissection of the polytene salivary gland nu clei in which the gene is transcribed. Here we analyze the order of in tron excision in relation to transcription and to intranuclear transpo rt. We demonstrate that the introns are excised with an overall 5' to 3' polarity that is established during transcription and maintained du ring transport. In contrast, we also show that individual introns are excised at very different rates and that neighboring introns are remov ed in a preferred order that is not necessarily 5' to 3'. Splicing fac tors are, in addition, shown to associate with the nascent BR3 pre-mRN A. Our data argue that functional spliceosomes assemble rapidly as int rons appear in the pre-mRNA, but that intron-specific properties influ ence the kinetics of spliceosome assembly and/or function, resulting i n cotranscriptional excision of some introns, preferentially those loc ated in the 5' part of the pre-mRNA, and posttranscriptional excision of other introns, preferentially those located in the 3' part of the p re-mRNA.