Distal regulatory regions of the rat MRF4 gene.

MRF4 is a muscle-specific transcription factor that is expressed both in embryonic somites and later in fetal and adult muscle fibers. Cis-regulatory elements of the MRF4 gene responsible for its complex expression pattern have not yet been identified, although previous studies of the rat MRF4 gene have demonstrated the presence of enhancer activity located several kilobases 5' to the transcription start site. Using cell transfection assays in vitro, we have now localized one of the regulatory regions of MRF4 to a 590-base-pair sequence between 4 and 5 kilobases upstream from the start site. This sequence region functioned as an enhancer in combination either with the MRF4 promoter or with the viral thymidine kinase (tk) promoter. Deletion analysis of MRF4 indicated the existence of another regulatory region, closer to the promoter, which functioned as an enhancer in combination with the MRF4 promoter but not with the tk promoter.