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 adenovi rus major late promoter (MLP) can be analyzed genetically in its corre ct location, despite its essential role in the viral life cycle, This genetic approach was extended to investigate the in vivo role of the i nitiator (INR), a transcriptional element that surrounds the start sit e of transcription, The analysis was designed to investigate if the IN R is an alternative basal element to the canonical TATA box of the MLP , its relative importance in the functioning of the promoter, and if i ts function was affected by upstream activating elements, Accordingly, two different mutations in the INR were created and tested in the gen ome, either by themselves or together with mutations in the TATA box o r one of the two upstream activating elements, the upstream promoter e lement (UPE) and the inverted CAAT box. The mutant viruses were examin ed first in one-step growth experiments, and then levels of late mRNA accumulation were measured by primer extension, transcription initiati on was assayed in isolated nuclei, and viral DNA accumulation was dete rmined by Southern hybridization. Neither mutation in the INR alone ha d any discernible phenotypic effects but when coupled to a phenotypica lly silent mutation in the TATA box gave rise to viruses with growth d efects that were attributable to a significantly lowered rate of trans cription initiation from the MLP. These results suggest that the INR p lays 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 phe notypically wild type, unlike the previously described TATA box-CAAT b ox double mutant, which had a severe transcription deficiency. Taken t ogether, the present and previous genetic results can be interpreted a s evidence that in the MLP, the TATA box and the UPE are the more impo rtant of the two basal and activating elements, respectively, but that the INR and CAAT can function in transcription initiation. We conside r the role of the INR in the formation of the preinitiation complex an d speculate on possible protein-protein interactions.