REVERSION OF A SIMIAN-VIRUS-40 ENHANCER MUTANT DEPENDS ON THE GROWTH-CONDITIONS

The simian virus 40 (SV40) enhancer contains three 8-bp purine-pyrimid ine (R:Y) alternating sequences (Z-motifs) which are known to adopt th e left-handed Z-DNA conformation in vitro. Mutations at these three Z- motifs seriously impair enhancer function. Reversion of one of these m utants (dpm12) is studied in this paper. The results indicate that, de pending on growth conditions, recovery of the enhancer function is ach ieved through different mechanisms. Mutant viruses growing in solid-ag ar medium do not revert. On the other hand, revertants obtained in liq uid medium contain a duplication of the enhancer sequences, showing no recovery of the original Z-motifs.