REGULATION OF ADENOVIRUS ALTERNATIVE RNA SPLICING AT THE LEVEL OF COMMITMENT COMPLEX-FORMATION

The adenovirus late region 1 (L1) represents an example of an altenati vely spliced gene where one 5' splice site is spliced to two alternati ve 3' splice sites, to produce two mRNAs; the 52,55K and IIIa mRNAs, r espectively. Accumulation of the L1 mRNAs is temporally regulated duri ng the infectious cycle. Thus, the proximal 3' splice site (52,55K mRN A) is used at all times during the infectious cycle whereas the distal 3' splice site (IIIa mRNA) is used exclusively late in infection. Her e we show that in vitro splicing extracts prepared from late adenoviru s-infected cells reproduces the virus-induced temporal shift from prox imal to distal 3' splice site selection in L1 pre-mRNA splicing. Two s table intermediates in spliceosome assembly have been identified; the commitment complex and the prespliceosome (or A complex). We show that the transition in splice site activity in L1 alternative splicing res ults from an increase in the efficiency of commitment complex formatio n using the distal 3' splice site in extracts prepared from late virus -infected cells combined with a reduction of the efficiency of proxima l 3' splice site splicing. The increase in commitment activity on the distal 3' splice site is paralleled by a virus-induced increase in A c omplex formation on the distal 3' splice site. Importantly, the virus- induced shift from proximal to distal L1 3' splice site usage does not require cis competition between the 52,55K and the IIIa 3' splice sit es, but rather results from the intrinsic property of the two 3' splic e sites which make them respond differently to factors in extracts pre pared from virus-infected cells.