Lack of homology between two haloacetate dehalogenase genes encoded on a plasmid from Moraxella sp. strain B

Abstract

Two genes encoding haloacetate dehalogenases, H-1 and H-2, are closely linked on a plasmid from Moraxella sp. strain B. H-1 predominantly acts on fluoroacetate, but H-2 does not. To elucidate the molecular relationship between the two enzymes, we compared their structural genes. Two restriction fragments of the plasmid DNA were subcloned on M13 phages and their nucleotide sequences were determined. The sequence of each fragment contained an open reading frame that was identified as the structural gene for each of the two dehalogenases on the basis of the following criteria; N-terminal amino acid sequence, amino acid composition, and molecular mass. The genes for H-1 and H-2, designated dehH1 and dehH2, respectively, had different sizes (885 bp and 675 bp) and G + C contents (58.3% and 53.4%). Sequence analysis revealed no homology between the two genes. We concluded that the dehalogenases H-1 and H-2 have no enzyme-evolutionary relationship. The deduced amino acid sequence of the dehH1 gene showed significant similarity to those of three hydrolases of Pseudomonas putida and a haloalkane dehalogenase of Xanthobacter autotrophicus. The dehH2 coding region was sandwiched between two repeated sequences about 1.8 kb long, which might play a part in the frequent spontaneous deletion of dehH2 from the plasmid.