Mutational analyses of the Moloney murine sarcoma virus enhancer.

Retroviral enhancers seem to play an integral role in determining tissue tropism. In an attempt to define the sequences essential for Moloney murine sarcoma virus (Mo-MSV) enhancer activity, we have undertaken deletion and insertion mutational analyses of the 72-bp repeats. We demonstrate that, in contrast to simian virus 40 (SV40), the proximal 19 bp of the 72-bp unit of Mo-MSV are not essential for enhancer function in mouse fibroblasts. Our analysis further localizes the 5' boundary of the Mo-MSV enhancer to a 5'-CAGGAT-3' region.