CHARACTERIZATION OF AN RNP COMPLEX THAT ASSEMBLES ON THE ROUS-SARCOMAVIRUS NEGATIVE REGULATOR OF SPLICING ELEMENT

We have characterized an RNP complex that assembles in nuclear extract s on the negative regulator of splicing (NRS) element from Rous sarcom a virus. While no complex was detected by native gel electrophoresis u nder conditions that supported spliceosome assembly, gel filtration re vealed a specific ATP-independent complex that rapidly assembled on NR S RNA, No complexes were formed on non-specific RNA, Unlike the non-sp ecific H complex, factors required for NRS complex assembly are limiti ng in nuclear extract, The NRS complex was not detected in reactions c ontaining ATP and pre-formed complexes were dissociated in the presenc e of ATP, In addition, the assembly process was sensitive to high salt but NRS complexes were salt stable once formed. Assembly of the NRS c omplex appears functionally significant since mutated NRS RNAs that fa il to inhibit splicing in vivo are defective for NRS complex assembly in nuclear extract, The probable relationship of the NRS complex to sp liceosomal complexes is discussed.