COOPERATIVE INTERACTIONS BETWEEN THE CAENORHABDITIS-ELEGANS HOMEOPROTEINS UNC-86 AND MEC-3

The POU-type homeodomain protein UNC-86 and the LIM-type homeodomain p rotein MEC-3, which specify neuronal cell fate in the nematode Caenorh abditis elegans, bind cooperatively as a heterodimer to the mec-3 prom oter. Heterodimer formation increases DNA binding stability and, there fore, increases DNA binding specificity. The in vivo significance of t his heterodimer formation in neuronal differentiation is suggested by (i) a loss-of-function mec-3 mutation whose product in vitro binds DNA well but forms heterodimers with UNC-86 poorly and (ii) a mec-3 mutat ion with wild-type function whose product binds DNA poorly but forms h eterodimers well.