Abstract
In Escherichia coli, two pathways use NADPH to reduce disulfide bonds
that form in some cytoplasmic enzymes during catalysis: the thioredoxin
system, which consists of thioredoxin reductase and thioredoxin,
and the glutaredoxin system, composed of glutathione reductase,
glutathione, and three glutaredoxins. These systems may also reduce
disulfide bonds which form spontaneously in cytoplasmic proteins
when E. coli is grown aerobically. We have investigated the role
of both systems in determining the thiol-disulfide balance in the
cytoplasm by determining the ability of protein disulfide bonds
to form in mutants missing components of these systems. We find
that both the thioredoxin and glutaredoxin systems contribute to
reducing disulfide bonds in cytoplasmic proteins. In addition, these
systems can partially substitute for each other in vivo since double
mutants missing parts of both systems generally allow substantially
more disulfide bond formation than mutants missing components of
just one system. Some of these double mutants were found to require
the addition of a disulfide reductant to the medium to grow well
aerobically. Thus, E. coli requires either a functional thioredoxin
or glutaredoxin system to reduce disulfide bonds which appear after
each catalytic cycle in the essential enzyme ribonucleotide reductase
and perhaps to reduce non-native disulfide bonds in cytoplasmic
proteins. Our results suggest the existence of a novel thioredoxin
in E. coli.
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