SEQUENCE-MEDIATED REGULATION OF ADENOVIRUS GENE-EXPRESSION BY REPRESSION OF MESSENGER-RNA ACCUMULATION

Gene expression in complex transcription units can be regulated at vir tually every step in the production of mature cytoplasmic mRNA, includ ing transcription initiation, elongation, termination, pre-mRNA proces sing, nucleus-to-cytoplasm mRNA transport, and alterations in mRNA sta bility, We have been characterizing alternative poly(A) site usage in the adenovirus major late transcription unit (MLTU) as a model for reg ulation at the level of pre-mRNA 3'-end processing, The MLTU contains five polyadenylation sites (L1 through L5), The promoter proximal site (L1) functions as the dominant poly(A) site during the early stage of adenovirus infection and in plasmid transfections when multiple poly( A) sites are present at the 3' end of a reporter plasmid, In contrast, stable mRNA processed at all five poly(A) sites is found during the l ate stage of adenovirus infection, after viral DNA replication has beg un. Despite its dominance during early infection, L1 is a comparativel y poor substrate for 3' end RNA processing both in vivo and in vitro, In this study we have investigated the basis for the early L1 dominanc e, We have found that mRNA containing an unprocessed L1 poly(A) site i s compromised in its ability to enter the steady-state pool of stable mRNA, This inhibition, which affects either the nuclear stability or n ucleus-to-cytoplasm transport of the pre-mRNA, requires a cis-acting s equence located upstream of the L1 poly(A) site.