Characterization of the single-strand-specific BPV-1 origin binding protein, SPSF I, as the HeLa Purα factor

Abstract

SPSF I and II are two cellular proteins which bind specifically to single-stranded DNA, SPSF I and II binding sites are found in the minimal origin of replication of BPV-1 DNA and near the P2 promoter of the cellular c-myc gene. DNA-binding properties of the two proteins to single-stranded oligonucleotides of different lengths and sequences were quantified by determination of DNA-binding constants. The binding constant of SPSF proteins to the lower strand of the BPV-1 origin was determined to be 1.5 x 10-10 M-1. Peptide sequences derived from purified SPSF I and II revealed the identity of at least one of the SPSF proteins with the so-called HeLa Purα factor. The HeLa Purα factor was identified previously by virtue of its capacity to bind to purine-rich strands of the PUR element found in initiation zones of DNA replication. Expression of the Pur cDNA confirmed the identity of the Puree protein with the 42 kDa SPSF I protein. Analysis of several Puree cDNA clones revealed the existence of an extended 3'-untranslated region in all Pur mRNAs.