Mutations in the adenovirus major late promoter: effects on viability and transcription during infection

We developed an experimental system to examine the effects of mutations in the adenovirus major late promoter in its correct genomic location during a productive infection. A virus was constructed whose genome could be digested to give a rightward terminal DNA fragment extending from the XhoI site at 22.9 map units, which can be ligated or recombined with plasmid DNA containing adenovirus sequences extending from 0 to 22.9 or 26.5 map units, respectively. Mutations were made by bisulfite mutagenesis in the region between base pairs -52 and -12 with respect to the cap site at +1 and transferred to the appropriate plasmids for viral reconstruction. Of 19 mutant plasmid sequences containing single or multiple G-to-A transitions, 14 could be placed in the viral genome with no apparent change in phenotype. These mutant sequences included those which contained four transitions in the string of G residues immediately downstream of the TATA box. There were no alterations in rates of transcription from the major late promoter, sites of transcription initiation, or steady-state levels of late mRNAs. All of the five mutant sequences which could not be placed in virus contained multiple transitions both up- and downstream of the TATA box. Two of these apparently lethal mutant sequences were used in promoter fusion experiments to test their ability to promote transcription of rabbit beta-globin sequences placed in the dispensable E1 region of the virus. Both sequences showed diminished ability compared with wild-type sequences to promote transcription in this context. Comparisons between these two sequences and the viable mutant sequences suggest a role for the string of G residues located between -38 and -33 in promoting transcription from the major late promoter. The data as a whole also demonstrate that the specific nucleotide sequence of this region of the major late promoter, which overlaps transcription elements of the divergent IVa2 transcription unit and coding sequences of the adenovirus DNA polymerase, is not rigidly constrained but can mutate extensively without loss of these several functions.