Purification of PO-B, a protein that has increased affinity for the pro-opiomelanocortin gene promoter after dephosphorylation

The human DNA-binding protein, PO-GA, is homologous to the large subunit of mouse replication factor C: regulation by alternate 3' processing of mRNA

We have previously cloned a human gene encoding a 128-kDa protein which we termed PO-GA [Lu et al., Biochem. Biophys. Res. Commun. 193 (1993) 779-786]. In the present report, we compared PO-GA to recent DNA database entries and determined that PO-GA was 80% identical, at the amino-acid level, to the large subunit of replication factor C (activator 1) cloned from mouse [Burbelo et al., Proc. Natl. Acad. Sci. USA 91 (1994) in press]. This indicates that PO-GA probably represents the corresponding subunit of human replication factor C. In addition, PO-GA has high homology to a putative Drosophila transcription factor. All three proteins contain a nuclear translocation signal and an ATP/ADP-binding motif, and are highly conserved in regions with homology to Escherichia coli and yeast DNA ligases. We determined that PO-GA mRNA species of 5.3 and 4.5 kb can be detected in most human tissues, but levels are especially high in ovary. Analysis of the sequence of a new PO-GA cDNA clone that we obtained reveals a previously undetected 650-bp 3'-UTR extension. This region contains several A+U-rich regions potentially involved in regulation of mRNA stability. This fragment only hybridizes to the larger 5.3-kb mRNA. Comparison of cDNA sequences revealed that the two mRNA species arise as a result of alternate use of poly(A)-addition sites. Since the ratio of the two mRNA species is variable in different tissues, we speculate that alternative 3' processing of the PO-GA mRNA is utilized as a mechanism of regulating cellular levels of mRNA.

Functional analysis of the cell-specific enhancer in the human proopiomelanocortin gene by beta-galactosidase histochemical staining

Nucleotide sequences responsible for the cell-specific expression of the human proopiomelanocortin (POMC) gene were analyzed by histochemical staining of beta-galactosidase in culture cells transfected with chimeric genes containing the 5'-flanking regions of the human POMC gene fused to the Escherichia coli lacZ gene. The chimeric genes were stably introduced into various culture cells, including AtT-20 cells, which express the endogenous mouse POMC gene. Whereas the control gene containing the cytomegalovirus enhancer was expressed in all cell lines tested, only AtT-20 cells supported the efficient transcription of the gene containing 2.9 kb of the human POMC 5'-flanking region. These results indicate that the stable transfection-expression system utilizing the histochemical detection of the gene expression is a useful method for the analysis of cell-specific gene expression. These results have also confirmed that the trans-acting factors in mouse AtT-20 cells interact with the human POMC gene promoter region and activate the transcription of the gene. Deletion analysis has demonstrated that the profiles of the transcriptional activity of the various human POMC-lacZ fusion genes are similar to those of the rat POMC gene described previously. Comparison of the human and the rat 5'-flanking sequences revealed close homology in several regions, which might be involved in the efficient transcription of the POMC gene in AtT-20 cells.