Cloning and characterization of the uvrD gene from an extremely thermophilic bacterium, Thermus thermophilus HB8.

The uvrD gene encodes a DNA helicase which plays an important role in prokaryotic nucleotide (nt) excision repair, mismatch repair and DNA replication. A cosmid-based genomic DNA library for Thermus thermophilus (Tt) HB8 was constructed, and this was screened by Southern hybridization using a uvrD fragment amplified by PCR as the probe. The nt sequence of cloned Tt uvrD was then determined. Characteristic helicase motifs, made up of seven elements, were all conserved in the amino acid (aa) sequence of Tt UvrD. The aa sequence showed 41% homology with that of Escherichia coli (Ec). In the aa composition of Tt UvrD, the number of Asn, Gln, Met and Cys residues was decreased, and the number of Pro residues was increased. The distribution of Pro residues and recent data on X-ray crystallographic structure suggested the importance of the structural dynamics of the protein. These changes are thought to stabilize the native protein conformation against heat denaturation. Tt uvrD complemented the UV sensitivity of a Ec uvrD mutant. Thus, the thermophilic bacterium has a UvrD helicase, whose function is common to Ec UvrD.