Journal of Bacteriology, April 2000, p. 2134-2141, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
andMarine Biotechnology Institute, Kamaishi Laboratories, Kamaishi, Iwate 026-0001, Japan
Received 8 October 1999/Accepted 18 January 2000
Nocardioides sp. strain KP7 grows on phenanthrene but
not on naphthalene. This organism degrades phenanthrene via
1-hydroxy-2-naphthoate, o-phthalate, and protocatechuate.
The genes responsible for the degradation of phenanthrene to
o-phthalate (phd) were found by Southern
hybridization to reside on the chromosome. A 10.6-kb DNA fragment
containing eight phd genes was cloned and sequenced. The
phdA, phdB, phdC, and
phdD genes, which encode the
and
subunits of the
oxygenase component, a ferredoxin, and a ferredoxin reductase,
respectively, of phenanthrene dioxygenase were identified. The gene
cluster, phdAB, was located 8.3 kb downstream of the previously characterized phdK gene, which encodes
2-carboxybenzaldehyde dehydrogenase. The phdCD gene cluster
was located 2.9 kb downstream of the phdB gene. PhdA and
PhdB exhibited moderate (less than 60%) sequence identity to the
and
subunits of other ring-hydroxylating dioxygenases. The PhdC
sequence showed features of a [3Fe-4S] or [4Fe-4S] type of
ferredoxin, not of the [2Fe-2S] type of ferredoxin that has been
found in most of the reported ring-hydroxylating dioxygenases. PhdD
also showed moderate (less than 40%) sequence identity to known
reductases. The phdABCD genes were expressed poorly in
Escherichia coli, even when placed under the control of
strong promoters. The introduction of a Shine-Dalgarno sequence upstream of each initiation codon of the phdABCD genes
improved their expression in E. coli. E. coli
cells carrying phdBCD or phdACD exhibited no
phenanthrene-degrading activity, and those carrying phdABD
or phdABC exhibited phenanthrene-degrading activity which
was significantly less than that in cells carrying the
phdABCD genes. It was thus concluded that all of the
phdABCD genes are necessary for the efficient expression of
phenanthrene-degrading activity. The genetic organization of the
phd genes, the phylogenetically diverged positions of these
genes, and an unusual type of ferredoxin component suggest phenanthrene
dioxygenase in Nocardioides sp. strain KP7 to be a new
class of aromatic ring-hydroxylating dioxygenases.
Present address: Shiseido Research Center, Kohoku-ku, Yokohama,
Kanagawa 223-0057, Japan.
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