Functional analysis of the role of the A + T-rich region and upstream flanking sequences in simian virus 40 DNA replication.

One boundary of the minimal origin of replication of simian virus 40 DNA lies within the A + T-rich region. Deletion of only a few bases into the adenine-thymine (AT) stretch results in a DNA template which is defective for replication both in vivo and in vitro (B. Stillman, R. D. Gerard, R. A. Guggenheimer, and Y. Gluzman, EMBO J. 4:2933-2939, 1985). In the present study, such deletion mutations have been reconstructed into a simian virus 40 genome containing an intact early promoter-enhancer region. The resulting mutants synthesized wild-type levels of T antigen, but were defective for replication and would not form plaques on CV-1 monkey cells. Replication-competent phenotypic revertants were selected after transfection of large quantities of the replication-defective viral DNAs into CV-1 cells. DNA sequence analysis showed that most of these revertants contained insertions or point mutations which partially regenerate the length of the AT stretch. These genotypic alterations were shown to be responsible for the revertant phenotype by replication analysis in vivo of subcloned revertant origin fragments. In general, our results emphasize the importance of the AT region to simian virus 40 origin function. However, one revertant retained the altered AT region but deleted six nucleotides upstream. Experiments using this mutant indicate that the 21-base-pair repeats identified as part of the early transcriptional promoter may compensate for defects in simian virus 40 DNA replication in vivo caused by mutations in the A + T-rich region when positioned at an appropriate distance from the core origin.