Purα activates PDGF-A gene transcription via interactions with a G-rich, single-stranded region of the promoter

Transcription of the PDGF-A chain gene is regulated by multiple promoter and silencer elements that are GC-rich and exhibit considerable single-stranded character. In this study, the 42 kDa single-stranded DNA and RNA binding protein, Purα, was investigated with respect to its ability to bind and interact functionally with single-stranded DNA elements in the PDGF-A gene. Recombinant GST-Purα bound with high affinity and sequence-specificity to the G-rich strands of two such transcriptional control elements, the 5′-S1 nuclease-hypersensitive silencer (5′SHS; −1418 to −1388) and the nuclease-hypersensitive element (NHE; −92 to −48). Ethylation interference footprinting localized binding of Purα to a region between nucleotides −91 and −77 within the NHE element, which contains binding sites for the double-stranded DNA-binding transcription factors Sp1, EGR-1 and WT1. Forced expression of Purα upregulated transcriptional activity of the PDGF-A promoter but not the 5′SHS silencer in HepG2 cells, demonstrating Purα has the potential to activate PDGF-A gene expression. Targeted disruption of the Purα gene reduced NHE activity and PDGF-A mRNA expression in mouse embryo fibroblasts, consistent with a physiological role for Purα in maintaining optimal transcription of the PDGF-A gene. These results indicate Purα enhances transcription of the PDGF-A gene through its interactions with single-stranded, G-rich strands in the promoter, perhaps by stabilizing non-B-form DNA conformations.