ORGANIZATION AND MYOGENIC RESTRICTED EXPRESSION OF THE MURINE SERUM RESPONSE FACTOR GENE - A ROLE FOR AUTOREGULATION

Serum response factor (SRF), a member of an ancient family of DNA-bind ing proteins, is generally assumed to be a ubiquitous transcription fa ctor involved in regulating growth factor-responsive genes, However, a vian SRF was recently shown (Croissant, J. D., Kim, J.-H., Eichele, G. , Goering, L., Lough, J., Prywes, R., and Schwartz, R. J. (1996) Dev. Biol. 177, 250-264) to be preferentially expressed in myogenic lineage s and is required for regulating post-replicative muscle gene expressi on. Given the central importance of SRF for the muscle tissue-restrict ed expression of the striated alpha-actin gene family, we wanted to de termine how SRP might contribute to this muscle-restricted expression, sere we have characterized the murine SRF genomic locus, which has se ven exons interrupted by six introns, with the entire locus spanning 1 1 kilobases, Murine SRF transcripts were processed to two 3'-untransla ted region polyadenylation signals, yielding 4.5- and 2.5-kilobase mRN A species. Murine SRF mRNA levels were the highest in adult skeletal a nd cardiac muscle, but barely detected in Liver, lung, and spleen tiss ues, During early mouse development, in situ hybridization analysis re vealed enrichment of SRF transcripts in the myotomal portion of somite s, the myocardium of the heart, and the smooth muscle media of vessels of mouse embryos, Likewise, murine SRF promoter activity was tissue-r estricted, being 80-fold greater in primary skeletal myoblasts than il l Liver-derived HepG2 cells, In addition, SRF promoter activity increa sed 6-fold when myoblasts withdrew from the cell cycle and fused into differentiated myotubes. A 310-base pair promoter fragment depended up on multiple intact serum response elements in combination with Sp1 sit es for maximal myogenic restricted activity. Furthermore, cotransfecte d SRF expression vector stimulated SRF promoter transcription, whereas dominant-negative SRF mutants blocked SRF promoter activity, demonstr ating a positive role for an SRF-dependent autoregulatory loop.