Control of adenovirus major late gene expression at multiple levels.

Most late adenovirus (Ad) proteins are translated from mRNAs originating from the so-called major late transcription unit (MLTU). These mRNAs are grouped into five families (designated L1 to L5), where each family consists of mRNAs that have co-terminal 3' ends. We have used mutant and wild-type Ad infections to characterize levels at which major late gene expression is regulated. Our results suggest the existence of a novel intermediate stage during a lytic infection where mRNAs from regions L1 and L4 are selectively overexpressed compared to the L2, L3, and L5 mRNAs. Our data suggest that this RNA phenotype reflects the activity of the MLTU at a transient stage immediately following initiation of viral DNA replication. Early during an Ad infection only mRNA from region L1 accumulate. To efficiently accumulate mRNA from regions L1 and L5 both viral DNA replication and late protein synthesis were required. To allow for only viral DNA replication resulted in an extensive premature transcription termination and a preferential mRNA accumulation from regions L1 and L4. The surprising production of L4 mRNA under these conditions was not due to the activation of a novel r-strand promoter located in the vicinity of region L4 or due to a control at the level of RNA transport or stability. Instead our results indicate that in the absence of efficient late protein synthesis 3' end formation occurs preferentially at the L1 and L4 poly(A) addition sites.