A novel growth-related nuclear protein binds and inhibits rat aldolase B gene promoter.

The promoter of the rat aldolase B (AldB) gene that confers liver-specific transcription has an additional role. It functions in vivo as an origin region of DNA replication in the cells in which the gene is repressed (Zhao, Y., Tsutsumi, R., Yamaki, M., Nagatsuka, N., Ejiri, S., Tsutsumi, K., 1994. Initiation zone of DNA replication at the rat aldolase B locus encompasses transcription promoter region. Nucleic Acids Res. 22, 5385-5390). This promoter/origin region has multiple protein-binding sites and, thus, binding of a particular set of protein factors in AldB-expressing or non-expressing cells seems to correlate with functional switch of this promoter/origin region. In the present study, we characterized two closely related proteins, termed AlF-C1 and AlF-C2, which are assumed to be involved in repression of the AldB gene. These two proteins share an identical amino acid sequence except for a 47-residue-insertion in AlF-C1, and are members of a gene family including heterogeneous nuclear ribonucleoprotein (hnRNP) and CCAAT-binding factor subunit A (CBF-A) genes. Bacterially expressed AlF-C1 can bind sequence-specifically to the AldB gene promoter, whereas AlF-C2 can only weakly. Transfection experiments using mammalian expression vectors showed that AlF-C1 down-regulates the AldB gene promoter in rat hepatoma cells, while AlF-C2 had no or little effect. Expressions of mRNAs encoding these two proteins are enriched in fetal livers and in regenerating livers. These results implied that AlF-C1 and/or C2 is involved in growth-regulated repression of the AldB gene.