Molecular genetic basis of allelic polymorphism in malate dehydrogenase (mdh) in natural populations of Escherichia coli and Salmonella enterica.

摘要

Nucleotide sequences of the mdh gene encoding themetabolic enzyme malate dehydrogenase (MDH) were determined for 44 strainsrepresenting the major lineages of Escherichia coli and the eight subspecies ofSalmonella enterica. Sequence diversity was four times greater in S. entericathan in E. coli, and in both species the rate of amino acid substitution waslower in the NAD(+)-binding domain than in the catalytic domain. Divergence ofthe mdh genes of the two species apparently has not involved excessnonsynonymous substitutions resulting from the fixation of adaptive amino acidmutations. Allozyme analysis detected 57% of the distinctive amino acidsequences. Statistical tests of the distribution of polymorphic synonymousnucleotide sites identified four possible intragenic recombination events, oneinvolving a single allele of E. coli and three involving alleles of the threesubspecies of S. enterica. But recombination at mdh has not occurred withsufficient frequency to obscure the phylogenetic relationships among strainsindicated by multilocus enzyme electrophoresis, total DNA hybridization, andsequence analysis of the gapA and putP genes. These findings provide furtherevidence that the effective (realized) rates of horizontal transfer andrecombination for metabolic enzyme and other housekeeping genes are generallylow in these species, in contrast to those for loci encoding or mediating thestructure of cell-surface and other macromolecules for which recombinants may besubject to strong balancing, directional, or diversifyingselection.