RNA polymerase II-dependent positional effects on mRNA 3 ' end processing in the adenovirus major late transcription unit

During the early phase of adenovirus infection, the promoter-proximal L1 po ly(A) site in the major late transcription unit is used preferentially desp ite the fact that the distal L3 poly(A) site is stronger (i.e. it competes better for processing factors and is cleaved at a faster rate, in vitro). P revious work had established that this was due at least in part to the stab le binding of the processing factor, cleavage and polyadenylation specifici ty factor, to the L1 poly(A) site as mediated by specific regulatory sequen ces. It is now demonstrated that in addition, the L1 poly(A) site has a pos itional advantage because of its 5' location in the transcription unit. We also show that preferential processing of a particular poly(A) site in a co mplex transcription unit is dependent on RNA polymerase II. Our results are consistent with recent reports demonstrating that the processing factors c leavage and polyadenylation specificity factor and cleavage stimulatory fac tor are associated with the RNA polymerase II holoenzyme; thus, processing at a weak poly(A) site like L1 can be enhanced by virtue of its being the f irst site to be transcribed.