Bipartite structure of the SGS1 DNA helicase in Saccharomyces cerevisiae

SGS1 in yeast encodes a DNA helicase with homology to the human BLM and WRN proteins. This group of proteins is characterized by a highly conserved DN A helicase domain homologous to Escherichia coli RecQ and a large N-termina l domain of unknown function. To determine the role of these domains in SGS 1 function, we constructed a series of truncation and helicase-defective (- hd) alleles and examined their ability to complement several sgs1 phenotype s Certain SGS1 alleles showed distinct phenotypes: sgs1-hd failed to comple ment the MMS hypersensitivity and hyper-recombination phenotypes, but parti ally complemnented the slow-growth suppression of top3 sgs1 strains and the OW-PI i,edly, an allele that encodes the amino terminus alone showed essent ially complete complementation of the hyper-recombination and top1 sgs1 def ects. In contrast, an allele encoding the helicase domain alone was unable to complement any sgs1 phenotype. Small truncations of the N ter-minus resu lted in hyper recombination and slow-growth phenotypes in excess of die nul l allele. These hypermorphic phenotypes could be relieved by deleting more of the N terminus, or in some cases, by a point mutation in the helicase do main. Intragenic complementation experiments demonstrate that both the amin o terminus and the DNA helicase are required for full SGS1 function. We con clude that the amino terminus of Sgs1 has an essential role in SGS1 functio n, distinct from that of the DNA helicase, with which it genetically inter acts.