Stable growth of simian virus 40 recombinants containing multimerized enhancers

Multiple copies of each of three genetically defined simian virus 40 protoenhancers, A, B, and C, were able to substitute for the wild-type simian virus 40 enhancer. Although the recombinant viruses grew poorly, they could be propagated without the accumulation of enhancer rearrangements that might improve viability. Mutations that inactivate the multimerized B and C protoenhancers abolished virus growth, but, unexpectedly, a mutation that inactivates the octamer-enhanson within the B protoenhancer increased virus viability. This positive effect may reflect loss of repression of the B protoenhancer by the ubiquitous octamer-motif-binding protein Oct-1.