Bacillus subtilis bacteriophage SPP1 G40P helicase lacking the N-terminal domain unwinds DNA bidirectionally

Bacillus subtilis bacteriophage SPP1 G40P hexameric replicative DNA helicase unidirectionally translocates with a 5'-> 3' polarity while separating the DNA strands. A G40P mutant derivative lacking the N-terminal domain (containing amino acid residues 110-442 from G40P, G40P Delta N109) was purified and characterized. G40P Delta N109 showed an ATPase activity that was dependent on the presence of single-stranded (ss) DNA. Unlike G40P, G40P Delta N109 was shown to bind with similar affinity both ssDNA arms of forked structures by nuclease protection assays. In a pH-dependent manner, G40P Delta N109 unwound a branched double-arm substrate preferentially with a 3'-> 5' polarity. Our results show that the linker region and the C-terminal domain of G40P are sufficient to render an enzyme capable of encircling the ssDNA tails of the forked DNA and to unwind DNA with both 5'-> 3' and 3'-> 5' polarity. The presence of the N-terminal domain, which does not play an essential role in helicase action, might be required indirectly for strand discrimination and polarity of translocation. (c) 2005 Elsevier Ltd. All rights reserved.