The yeast STM1 gene encodes a purine motif triple helical DNA-binding protein

The formation of triple helical DNA has been evoked in several cellular pro cesses including transcription, replication, and recombination. Using conve ntional and affinity chromatography, we purified from Saccharomyces cerevis iae whole-cell extract a 35-kDa protein that avidly and specifically bound a purine motif tripler (with a K-d of 61 pM) but not a pyrimidine motif tri plex or duplex DNA. Peptide microsequencing identified this protein as the product of the STM1 gene. Confirmation that Stm1p is a purine motif tripler -binding protein was obtained by electrophoretic mobility shift assays usin g either bacterially expressed, recombinant Stm1p or whole-cell extracts fr om stm1 Delta yeast. Stm1p has previously been identified as G4p2, a G-quar tet nucleic acid-binding protein. This suggests that some proteins actually recognize features shared by G4 DNA and purine motif triplexes, e.g. Hoogs teen hydrogen-bonded guanines. Genetically, the STM1 gene has been identifi ed as a multicopy suppressor of mutations in several genes involved in mito sis (e.g. TOM1, MPT5, and POP2). A possible role for multiplex DNA and its binding proteins in mitosis is discussed.