We showed previously that rad50 and mre11 genes of thermophilic archaea are organized in an operon-like structure with a third gene (nurA) encoding a 5' to 3' exonuclease. Here, we show that the rad50, mre11 and nurA genes from the hyperthermophilic archaeon Sulfolobus acidocaldarius are co-transcribed with a fourth gene encoding a DNA helicase. This enzyme (HerA) is the prototype of a new class of DNA helicases able to utilize either 3' or 5' single-stranded DNA extensions for loading and subsequent DNA duplex unwinding. To our knowledge, DNA helicases capable of translocating along the DNA in both directions have not been identified previously. Sequence analysis of HerA shows that it is a member of the TrwB, FtsK and VirB4/VirD4 families of the PilT class NTPases. HerA homologs are found in all thermophilic archaeal species and, in all cases except one, the rad50, mre11, nurA and herA genes are grouped together. These results suggest that the archaeal Rad50-Mre11 complex might act in association with a 5' to 3' exonuclease (NurA) and a bipolar DNA helicase (HerA) indicating a probable involvement in the initiation step of homologous recombination.
