Complex regulation of the muscle-specific contractile protein (troponin I) gene.

A cloned quail troponin I contractile protein gene, stably transfected into a mouse myogenic cell line, exhibits appropriate developmental activation and quantitative expression during myoblast differentiation. Deletion mutagenesis analyses reveal that the troponin I gene has two distinct cis regulatory elements required for its developmental expression, as measured by mRNA accumulation and nuclear runoff transcription assays. One element in the 5' flanking region is required for maximum quantitative expression, and a second larger regulatory element (1.5 kilobases) within the first intron is responsible for differentiation-specific transcription. The upstream region is highly sensitive to negative repression by interaction with pBR322 sequences. The larger intragenic region retains some activity when moved to the 5' and 3' flanking regions and when inverted but is maximally active in its native intragenic site. The concerted activities of these two regulatory regions produce a 100- to 200-fold transcriptional activation during myoblast differentiation. The conserved 5' exon-intron organization of troponin I and other contractile protein genes suggests a possible mechanism by which intragenic control elements coordinate contractile protein gene regulation during skeletal myogenesis.