Saccharomyces cerevisiae Hrql helicase activity is affected by the sequence but not the length of single-stranded DNA

Mutations in the human RecQ4 DNA helicase are associated with three different diseases characterized by genomic instability. To gain insight into how RecQ4 dysfunction leads to these pathologies, several groups have used the Saccharomyces cerevisiae RecQ4 homolog Hrql as an experimental model. Hrql displays many of the same functions as RecQ4 in vivo and in vitro. However, there is some disagreement in the literature about the effects of single-stranded DNA (ssDNA) length on Hrql helicase activity and the ability of Hrql to anneal complementary ssDNA oligonucleotides into duplex DNA. Here, we present a side-by-side comparison of Hrql and RecQ4 helicase activity, demonstrating that in both cases, long random-sequence 3' ssDNA tails inhibit DNA unwinding in vitro in a length-dependent manner. This appears to be due to the formation of secondary structures in the random-sequence ssDNA because Hrql preferentially unwound poly(dT)-tailed forks independent of ssDNA length. Further, RecQ4 is capable of ssDNA strand annealing and annealing-dependent strand exchange, but Hrql lacks these activities. These results establish the importance of DNA sequence in Hrql helicase activity, and the absence of Hrql strand annealing activity explains the previously identified discrepancies between S. cerevisiae Hrql and human RecQ4. (C) 2017 Elsevier Inc. All rights reserved.