Characterization of an RNP complex that assembles on the Rous sarcoma virus negative regulator of splicing element

We have characterized an RNP complex that assembles in nuclear extracts on the negative regulator of splicing (NRS) element from Rous sarcoma virus. While no complex was detected by native gel electrophoresis under conditions that supported spliceosome assembly, gel filtration revealed a specific ATP-independent complex that rapidly assembled on NRS RNA, No complexes were formed on non-specific RNA, Unlike the non-specific H complex, factors required for NRS complex assembly are limiting in nuclear extract, The NRS complex was not detected in reactions containing ATP and pre-formed complexes were dissociated in the presence of ATP, In addition, the assembly process was sensitive to high salt but NRS complexes were salt stable once formed. Assembly of the NRS complex appears functionally significant since mutated NRS RNAs that fail to inhibit splicing in vivo are defective for NRS complex assembly in nuclear extract, The probable relationship of the NRS complex to spliceosomal complexes is discussed.