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J. Bacteriol., 11 1995, 6545-6551, Vol 177, No. 22
PG Egland, J Gibson and CS Harwood
The first step of anaerobic benzoate degradation is the formation of
benzoyl-coenzyme A by benzoate-coenzyme A ligase. This enzyme, purified
from Rhodopseudomonas palustris, is maximally active with 5 microM
benzoate. To study the molecular basis for this reaction, the benzoate-
coenzyme A ligase gene (badA) was cloned and sequenced. The deduced amino
acid sequence of badA showed substantial similarity to other coenzyme A
ligases, with the highest degree of similarity being that to
4-hydroxybenzoate-coenzyme A ligase (50% amino acid identity) from R.
palustris. A badA mutant that was constructed had barely detectable levels
of ligase activity when cell extracts were assayed at 10 microM benzoate.
Despite this, the mutant grew at wild-type rates on benzoate under
laboratory culture conditions (3 mM benzoate), and mutant cell extracts had
high levels of ligase activity when assayed at a high concentration of
benzoate (1 mM). This suggested that R. palustris expresses, in addition to
BadA, a benzoate-activating enzyme(s) with a relatively low affinity for
benzoate. A possible role of 4- hydroxybenzoate-coenzyme A ligase (encoded
by hbaA) in this capacity was investigated by constructing a badA hbaA
double mutant. Although the double mutant grew more slowly on benzoate than
badA cells, growth rates were still significant, suggesting the involvement
of a third enzyme in benzoate activation. Competition experiments involving
the addition of a small amount of cyclohexanecarboxylate to ligase assay
mixtures implicated cyclohexanecarboxylate-coenzyme A ligase as being this
third enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)
Copyright © 1995, American Society for Microbiology
Benzoate-coenzyme A ligase, encoded by badA, is one of three ligases able to catalyze benzoyl-coenzyme A formation during anaerobic growth of Rhodopseudomonas palustris on benzoate
Department of Microbiology, University of Iowa, Iowa City 52242, USA.
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