2 NEW MEMBERS OF THE MURINE SIM GENE FAMILY ARE TRANSCRIPTIONAL REPRESSORS AND SHOW DIFFERENT EXPRESSION PATTERNS DURING MOUSE EMBRYOGENESIS

From a cDNA library of mouse skeletal muscle, we have isolated mouse S im1 (mSim1) cDNA encoding a polypeptide of 765 amino acids with striki ng amino acid identity in basic helix-loop-helix (89% identity) and PA S (89% identity) domains to previously identified mSim2, although the carboxy-terminal third of the molecule did not show any similarity to mSim2 or Drosophila Sim (dSim). Yeast two-hybrid analysis and coimmuno precipitation experiments demonstrated that both of the mSim gene prod ucts interacted with Amt even more efficiently than AhR, a natural par tner of Arnt, suggesting a functional cooperativity with Amt. In sharp contrast with dSim having transcription-enhancing activity in the car boxy-terminal region, the two mSims possessed a repressive activity to ward Arnt in the heterodimer complex. This is the first example of bHL H-PAS proteins with transrepressor activity, although some genetic dat a suggest that dSim plays a repressive role in gene expression (Z. Cha ng, D. Price, S. Bockheim, M. J. Boedigheimer, R. Smith, and A. Laugho n, Dev. Biol. 160:315-332, 1993; D. M. Mellerick and M. Nirenberg, Dev . Biol. 171:306-316, 1995). Whole-mount in situ hybridization showed r estricted and characteristic expression patterns of the two mSim mRNAs in various tissues and organs during embryogenesis, such as those for the somite, the nephrogenic cord, and the mesencephalon (for mSim1) a nd those for the diencephalon, branchial arches, and limbs (for mSim2) . From sequence similarity and chromosomal localization, it is conclud ed that mSim2 is an ortholog of hSIM2, which is proposed to be a candi date gene responsible for Down's syndrome. The sites of mSim2 expressi on showed an overlap with the affected regions of the syndrome, furthe r strengthening involvement of mSim2 in Down's syndrome.