DISTINGUISHABLE PATTERNS OF PROTEIN-DNA INTERACTIONS INVOLVING COMPLEXES OF BASIC HELIX-LOOP-HELIX PROTEINS

Myogenic factors and TAL1 possess distinguishable DNA binding characte ristics when they form a complex with basic helix- loop-helix (bHLH) p roteins of class A. These characteristics were evident in electrophore tic mobility shift assays showing that complexes of myogenic factors a nd HTF4 displayed a relatively high affinity for the enhancer in the m uscle creatine kinase gene, whereas TAL1 appeared to greatly attenuate the interaction of HTF4 with this enhancer. In addition, by forming a complex with HTE4 in solution, TAL1 could exert a negative effect on the interactions of HTF4 with elements that include E box motifs of mu E2 (CAGCTG) and kappa E2/mu E5 (CACCTG) type. Similarly, heterodimers containing TAL1 and the DNA binding domain of E47 exhibited a relativ ely weak affinity for mu E2 and kappa E2/mu E5 core motifs. The result s of both studies invoked the hypothesis that in vivo TAL1 might act a s a negative regulator of mu E2 and kappa E2/mu E5 sequence motifs by forming a complex with the products of the E2A and HTF4 genes. Support for this hypothesis was obtained by transient expression analyses whe re TAL1 was found to inhibit the activation effects produced by E2-5 a nd HTF4a on a reporter gene construct containing repeated mu E2 and mu E5 motifs, derived from the immunoglobulin gene enhancer.