CONDITIONALLY LETHAL ESCHERICHIA-COLI MUREIN MUTANTS CONTAIN POINT-DEFECTS THAT MAP TO REGIONS CONSERVED AMONG MUREIN AND FOLYL POLY-GAMMA-GLUTAMATE LIGASES - IDENTIFICATION OF A LIGASE SUPERFAMILY
Ss. Eveland et al., CONDITIONALLY LETHAL ESCHERICHIA-COLI MUREIN MUTANTS CONTAIN POINT-DEFECTS THAT MAP TO REGIONS CONSERVED AMONG MUREIN AND FOLYL POLY-GAMMA-GLUTAMATE LIGASES - IDENTIFICATION OF A LIGASE SUPERFAMILY, Biochemistry, 36(20), 1997, pp. 6223-6229
Bacterial peptidoglycan biosynthesis includes four enzymatic reactions
in which successive amino acid residues are ligated to uridine diphos
pho-N-acetylmuramic acid (UDP-MurNAc). By comparing the amino acid seq
uences of MurC, -D, -E, and -F proteins from various bacterial genera,
four regions of homology were identified. A profile search of Swisspr
ot for related sequences revealed that these regional similarities wer
e present in the folyl-gamma-polyglutamate ligases. These sequence hom
ologies appear to track with catalytic function: both enzyme families
proceed through an ordered kinetic mechanism and form product via an a
cyl phosphate intermediate. Two highly conserved residues in region II
were examined through site-directed mutagenesis of the murein D-alany
l-D-alanine-adding enzyme from Escherichia coli (murF; E158 and H188).
All mutations were highly detrimental to activity with enzyme specifi
c activity reductions of 200-4500-fold, validating the critical nature
of these residues. DNA sequence analysis from three E. coli mutants h
arboring the murC3 (G344D), murE1 (G344K, A495S), and murF2 (A288T) mu
tations revealed the presence of point mutation(s) closely associated
with the fourth of these aligned regions. The murF2 allele, expressed
and purified as a glutathione S-transferase::MurF2 fusion, was 181-fol
d less catalytically active at 30 degrees C and was further reduced at
the nonpermissive temperature (42 degrees C). Thus the murF2 temperat
ure-sensitive phenotype arises from a point mutation within a highly c
onserved region within this protein family. These data argue that thes
e proteins comprise a superfamily of three substrate amide ligases tha
t share significant structural and catalytic homologies.