Distinct rat proteins can recognize CCAAT-homologous sequences of the metallothionein promoter and trans-activate this promoter.

We have previously purified and characterized a rat liver protein C'BP-1 that is either identical or closely related to C/EBPdelta (Aniskovitch and Jacob, 1997). The mouse metallothionein-I (MT-I) promoter contains two C'BP-1 binding sites, one of which includes the MRE-c' region (-135 to -110). The C'BP-1 binding activity was detected by EMSA as a major activity for MRE-c' in nonproliferating adult liver cells but not in rat hepatoma cells. In this study, we purified and characterized a factor, C'BP-2, which had a dominant binding activity for MRE-c' in Morris hepatoma 3924A, a poorly differentiated, fast-growing tissue. C'BP-2 is a 28 kDa protein which exists in solution as a monomer. As observed for C'BP-1, affinity-purified C'BP-2 stimulated transcription from the mMT-I gene promoter. DNase I footprinting revealed two C'BP-2 binding sites in the regions that overlap with the CCAAT homologies of the C'BP-1 binding sites on the mMT-I promoter. C'BP-2 made essential contacts with the CCAAT homology and in the region upstream of this sequence. Competition electrophoretic mobility shift assay and methylation interference analysis revealed that C'BP-2 is a protein closely related, but not identical, to CP2. These data suggest that C'BP-1 and C'BP-2 may play a role in hepatocyte proliferation and/or differentiation.