Nuclear localization signal region in nuclear receptor PXR governs the receptor association with mitotic chromatin.

In recent years, some transcription factors have been observed to remain associated with mitotic chromatin. Based on these observations, it is suggested that these chromatin-bound transcription factors may serve as 'epigenetic marks' for transmission of pattern of gene expression from progenitor to progeny cells. In this context, our laboratory has reported that nuclear receptor PXR, a master regulator of xenobiotic metabolism, remains constitutively associated with mitotic chromatin. However, the region responsible for this interaction with chromatin remained unknown. In this study, we have shown, for the first time, that mitotic chromatin association of this factor is mediated by the combined action of two zinc fingers present in the DNA-binding domain of PXR. Overall, the nuclear localization signal (NLS) region appears to play a major role in this interaction with mitotic chromatin. Also, we have identified a sub-region of 11 amino acid residues within NLS region of PXR (R66-76R) essential for receptor interaction with the mitotic chromatin. Interestingly, this minimal region is sequence-specific and independent of its basic charge. We have termed this minimal sub-region as 'mitotic chromatin binding-determining region' (MCBR). It is suggested that this receptor region is essential for activation of its target genes. Additionally, we have shown that PXR remains associated with the everted repeat (ER6) region of its major target gene, CYP3A4 promoter during mitosis implying its suggested role in 'gene bookmarking'.