Unscheduled expression of capsid protein IIIa results in defects in adenovirus major late mRNA and protein expression

Adenovirus gene expression is to a large extent regulated at the level of alternative RNA splicing. For example, in the major late region 1 (L1) unit, a common 5′ splice site can be joined to two alternative 3′ splice sites, resulting in the formation of the so-called 52,55K (proximal 3′ splice site) or the IIIa (distal 3′ splice site) mRNAs. Whereas, the 52,55K mRNA is expressed both early and late during infection, the IIIa mRNA is strictly confined to the late phase of the infectious cycle. We have previously shown that IIIa mRNA splicing is subjected to a tight viral control of IIIa 3 splice site usage. In an attempt to determine why adenovirus uses elaborate mechanisms to confine IIIa mRNA production to the late phase of infection, we characterized the phenotype of a recombinant adenovirus expressing the IIIa protein from an inducible tetracycline regulated gene cassette. The results show that expression of the IIIa protein during the early phase of infection results in a significant reduction in late viral protein synthesis and a moderate block to viral DNA replication. Interestingly, unscheduled IIIa protein expression resulted in a perturbation of the accumulation of alternatively spliced L1 mRNAs. Thus, 52,55K mRNA accumulation was inhibited while no effects on endogenous IIIa mRNA expression was detected.