A novel TCTG(G/C) direct repeat and an A/T-rich HMG2-binding site control the expression of the rat cardiac troponin T gene.

We previously showed that the rat cardiac troponin T proximal promoter (-497bp from the transcriptional start site) was sufficient to confer cardiac-specific expression of a reporter gene in both cultured cardiomyocytes and transgenic mice. This promoter consists of two cis-regulatory modules with high sequence similarity. Nucleotides from -319 to -289 (termed D2) and nucleotides from -249 to -209 (termed F41) contain a TCTG(G/C) direct repeat and an A/T-rich sequence. Competition gel mobility shift assays revealed that the same protein factors were bound to both D2 and F41. Deletion and religation studies of the promoter suggested that D2 acted as an enhancer but could not totally substitute for the F41 function. Mutational analyses demonstrated that the direct repeat was required for the DNA-binding and promoter activity. Moreover, cardiac-specific 42kDa proteins and a ubiquitous high mobility group 2 (HMG2) protein were identified to be responsible for the binding to the TCTG(G/C) direct repeat and the A/T-rich sequence, respectively. Overexpression of HMG2 had differential effects on the promoter in cardiomyocytes versus fibroblasts. Our results provided the first evidence to support that HMG2 together with tissue-specific factors could direct a stimulatory or inhibitory effect on thecardiac troponin T gene expression in heart or non-heart tissue, respectively.