Methylation of simian virus 40 Hpa II site affects late, but not early, viral gene expression.

DNA methylation has been correlated with reduced gene expression in a number of studies, although evidence for a casual link between the two events has been lacking. Because microinjection of simian virus 40 (SV40) DNA into the nucleus of Xenopus laevis oocytes results in the synthesis of both early and late viral gene products, it was possible to test whether a specific methylation event can affect gene expression. The single SV40 Hpa II site at 0.72 SV40 map units was specifically methylated with Hpa II methylase. When this DNA was injected into oocytes, there was a marked reduction in the synthesis of the major late viral capsid protein VP-1, relative to the synthesis by an unmethylated control. However, production of the early proteins (the large and small tumor antigens) was not affected by Hpa II methylation. Therefore, methylation at a single site on the viral DNA located near the 5' end of the late region can specifically repress late gene expression. The possible mechanisms by which this repression is mediated are discussed.