A protein binding to CArG box motifs and to single-stranded DNA functions as a transcriptional repressor

Abstract

A CArG box motif [CC(A+T-rich) 6GG] is one of the DNA elements required for muscle-specific gene transcription. Nuclear factors in mouse C2 myogenic cells strongly bind to the CArG box in the first intron of the gene ( Sm α- A) encoding human smooth muscle α-actin. To clone cDNAs of the CArG box-binding factor (CBF), λgt11 cDNA expression libraries from C2 cells were screened for in situ DNA binding specific for this CArG box sequence. The 1.6-kb cDNA ( CBF-A) encoding 285 amino acids (aa) was obtained, and a β-galactosidase fusion protein, bacterially produced from the cDNA, bound to DNA fragments containing several CArG boxes. When the expression level of CBF-A in C2 cells increased by transfection of CBF-A expression plasmids, Sm α- A transcription was repressed. The deduced aa sequence of CBF-A is similar to some single-stranded (ss) nucleic acid-binding proteins. The fusion protein could bind to ssDNA, whereas CBF in C2 cell nuclear extracts could not. From these results, CBF-A is a novel CArG box-, ssDNA- and RNA-binding protein, as well as a repressive transcriptional factor.