RECQL5 helicase: Connections to DNA recombination and RNA polymerase II transcription

The RecQ family of helicases are traditionally viewed as recombination factors, important for maintaining genome stability. RECQL5 is unique among these proteins in being associated with RNA polymerase II, the enzyme responsible for transcribing all protein-encoding genes in eukaryotes. Here, we describe the possible implications of recent studies and discuss models for RECQL5 function.     

Functional role of the Werner syndrome RecQ helicase in human fibroblasts

Werner syndrome is an autosomal recessive human genetic instability and cancer predisposition syndrome that also has features of premature aging. We focused on two questions related to Werner syndrome protein (WRN) function in human fibroblasts: Do WRN‐deficient fibroblasts have a consistent cellular phenotype? What role does WRN play in the recovery from replication arrest? We identified consistent cell proliferation and DNA damage sensitivity defects in both primary and SV40‐transformed fibroblasts from different Werner syndrome patients, and showed that these defects could be revealed by acute depletion of WRN protein. Mechanistic analysis of the role of WRN in recovery from replication arrest indicated that WRN acts to repair damage resulting from replication arrest, rather than to prevent the disruption or breakage of stalled replication forks. These results identify readily quantified cell phenotypes that result from WRN loss in human fibroblasts; delineate the impact of cell transformation on the expression of these phenotypes; and define a mechanistic role for WRN in the recovery from replication arrest.     

A central role for SSB in Escherichia coli RecQ DNA helicase function

RecQ DNA helicases are critical components of DNA replication, recombination, and repair machinery in all eukaryotes and bacteria. Eukaryotic RecQ helicases are known to associate with numerous genome maintenance proteins that modulate their cellular functions, but there is little information regarding protein complexes involving the prototypical bacterial RecQ proteins. Here we use an affinity purification scheme to identify three heterologous proteins that associate with Escherichia coli RecQ: SSB (single-stranded DNA-binding protein), exonuclease I, and RecJ exonuclease. The RecQ-SSB interaction is direct and is mediated by the RecQ winged helix subdomain and the C terminus of SSB. Interaction with SSB has important functional consequences for RecQ. SSB stimulates RecQ-mediated DNA unwinding, whereas deletion of the C-terminal RecQ-binding site from SSB produces a variant that blocks RecQ DNA binding and unwinding activities, suggesting that RecQ recognizes both the SSB C terminus and DNA in SSB.DNA nucleoprotein complexes. These findings, together with the noted interactions between human RecQ proteins and Replication Protein A, identify SSB as a broadly conserved RecQ-binding protein. These results also provide a simple model that explains RecQ integration into genome maintenance processes in E. coli through its association with SSB.