The initiator element of the adenovirus major late promoter has an important role in transcription initiation in vivo.

Previous results showed that the structure and function of the adenovirus major late promoter (MLP) can be analyzed genetically in its correct location, despite its essential role in the viral life cycle. This genetic approach was extended to investigate the in vivo role of the initiator (INR), a transcriptional element that surrounds the start site of transcription. The analysis was designed to investigate if the INR is an alternative basal element to the canonical TATA box of the MLP, its relative importance in the functioning of the promoter, and if its function was affected by upstream activating elements. Accordingly, two different mutations in the INR were created and tested in the genome, either by themselves or together with mutations in the TATA box or one of the two upstream activating elements, the upstream promoter element (UPE) and the inverted CAAT box. The mutant viruses were examined first in one-step growth experiments, and then levels of late mRNA accumulation were measured by primer extension, transcription initiation was assayed in isolated nuclei, and viral DNA accumulation was determined by Southern hybridization. Neither mutation in the INR alone had any discernible phenotypic effects but when coupled to a phenotypically silent mutation in the TATA box gave rise to viruses with growth defects that were attributable to a significantly lowered rate of transcription initiation from the MLP. These results suggest that the INR plays a role in vivo and can act as an alternative basal element in the absence of a functioning TATA box. A virus with mutations in both the INR and the UPE, although viable, likewise had a severe deficiency in transcription, suggesting that the function of the INR is affected by that of the UPE. This contrasts with the previous report that a TATA box-UPE double mutation is not recoverable in virus. In addition, the virus with mutations in both the INR and the inverted CAAT box was phenotypically wild type, unlike the previously described TATA box-CAAT box double mutant, which had a severe transcription deficiency. Taken together, the present and previous genetic results can be interpreted as evidence that in the MLP, the TATA box and the UPE are the more important of the two basal and activating elements, respectively, but that the INR and CAAT can function in transcription initiation. We consider the role of the INR in the formation of the preinitiation complex and speculate on possible protein-protein interactions.