Molecular dissection of DNA sequences and factors involved in slow muscle-specific transcription

Transcription is a major regulatory mechanism for the generation of slow- a nd fast-twitch myofibers. We previously identified an upstream region of th e slow TnI gene (slow upstream regulatory element [SURE]) and an intronic r egion of the fast TnI gene (fast intronic regulatory element [FIRE]) that a re sufficient to direct fiber type-specific transcription in transgenic mic e. Here we demonstrate that the downstream half of TnI SURE, containing E b ox, NFAT, MEF-2, and CACC motifs, is sufficient to confer pan-skeletal musc le-specific expression in transgenic mice. However, upstream regions of SUR E and FIRE are required for slow and fast fiber type specificity, respectiv ely. By adding back upstream SURE sequences to the pan-muscle-specific enha ncer, we delineated a 15-bp region necessary for slow muscle specificity. U sing this sequence in a yeast one-hybrid screen, we isolated cDNAs for gene ral transcription factor 3 (GTF3)/muscle TFII-I repeat domain-containing pr otein 1 (MusTRD1). GTF3 is a multidomain nuclear protein related to initiat or element-binding transcription factor TF II-I; the genes for both protein s are deleted in persons with Williams-Beuren syndrome, who often manifest muscle weakness. Gel retardation assays revealed that full-length GTF3, as well as its carboxy-terminal half, specifically bind the bicoid-like motif of SURE (GTTAATCCG). GTF3 expression is neither muscle nor fiber type speci fic. Its levels are highest during a period of fetal development that coinc ides with the emergence of specific fiber types and transiently increases i n regenerating muscles damaged by bupivacaine. We further show that transcr iption from TnI SURE is repressed by GTF3 when overexpressed in electropora ted adult soleus muscles. These results suggest a role for GTF3 as a regula tor of slow TnI expression during early stages of muscle development and su ggest how it could contribute to Williams-Beuren syndrome.