Biochemical studies of the Saccharomyces cerevisiae Mph1 helicase on junction-containing DNA structures.

Saccharomyces cerevisiae Mph1 is a 3-5' DNA helicase, required for the maintenance of genome integrity. In order to understand the ATPase/helicase role of Mph1 in genome stability, we characterized its helicase activity with a variety of DNA substrates, focusing on its action on junction structures containing three or four DNA strands. Consistent with its 3' to 5' directionality, Mph1 displaced 3'-flap substrates in double-fixed or equilibrating flap substrates. Surprisingly, Mph1 displaced the 5'-flap strand more efficiently than the 3' flap strand from double-flap substrates, which is not expected for a 3-5' DNA helicase. For this to occur, Mph1 required a threshold size (>5 nt) of 5' single-stranded DNA flap. Based on the unique substrate requirements of Mph1 defined in this study, we propose that the helicase/ATPase activity of Mph1 play roles in converting multiple-stranded DNA structures into structures cleavable by processing enzymes such as Fen1. We also found that the helicase activity of Mph1 was used to cause structural alterations required for restoration of replication forks stalled due to damaged template. The helicase properties of Mph1 reported here could explain how it resolves D-loop structure, and are in keeping with a model proposed for the error-free damage avoidance pathway.