Several in vitro studies have suggested that high mobility group (HMG) protein 1 has a role in gene regulation as a trans activator or quasi-transcription factor. However, data on the molecular functions of HMG1 protein in these reactions are contradictory or obscure. In order to assess whether HMG1 protein does, in fact, have transcriptional activation potential, two assay systems in cultured cells were employed. HMG1 protein introduced into COS-1 cells as a complex with a reporter plasmid carrying the lacZ gene enhanced the level of the gene expression. Cotransfection of an expression plasmid carrying HMG1 cDNA into the cells with the reporter plasmid enhanced the activity of β-galactosidase 2-3-fold in comparison with that of the control effector plasmid. The enhancement was proved to be dependent not on the replication but on the transcription of the reporter plasmid. In the cotransfection experiments, an expression plasmid encoding the HMG1 molecule lacking the acidic carboxyl terminus repressed the expression of the reporter gene. The binding of an HMG1 protein variant lacking the acidic carboxyl terminus to DNA gave an extremely large shift of gel retardation in comparison with the complete HMG1 molecule. Together, these results indicate that HMG1 protein can enhance expression in cells in culture at the step of gene transcription and that the DNA binding domains comprising two-thirds of the HMG1 protein molecule are responsible for the inhibition property. Also, the acidic terminus of the HMG1 molecule is essential for the enhancement of gene expression in addition to elimination of the repression caused by the DNA binding. Thus it has been directly demonstrated that HMG1 protein can function as a quasi-transcription factor in the process of gene transcription. The repressive property of the HMG box in gene expression and the stimulation induced by the acidic region may help to shed light on the functional mechanism of the many transcription factors containing a DNA-binding motif and/or an acidic domain.
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