Two MAR DNA-binding proteins of the pea nuclear matrix identify a new class of DNA-binding proteins.

Four MAR-binding proteins of 60, 65, 70 and 72 kDa have been detected by South-Western blotting and isolated from pea nuclear matrices. Two cDNAs encoding the 60 and 65 kDa proteins (MARBP-1 and MARBP-2) were isolated from a pea leaf cDNA library by screening with a PCR product obtained using degenerate primers based on an amino acid sequence from the 60 kDa protein. The proteins of 560 and 550 amino acids are 86% identical and contain several KKD/E repeats near the C-terminus. Escherichia coli-expressed MARBP-1 specifically binds A/T-rich MAR DNA. The interaction of MARBP-1/MARBP-2 with MAR DNA involves novel DNA-binding motifs. The MARBP-1 and MARBP-2 genes are expressed in a range of pea tissues and are encoded by genes at different loci. MARBP-1 and MARBP-2 are homologous to yeast nucleolar proteins Nop56p and Nop58p, which are involved in ribosome biogenesis, and to similar highly conserved proteins in other eukaryotes and in archaebacteria. MARBP-1 and MARBP-2 may have multifunctional roles in chromatin organisation and ribosome biogenesis.