Affiliation:
1. Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Abstract
ABSTRACT
Among the rhizobia that establish nitrogen-fixing nodules on the roots of host plants, many contain multiple copies of genes encoding the sigma factor RpoH and the chaperone GroEL/GroES. In
Sinorhizobium meliloti
there are two
rpoH
genes, four
groESL
operons, and one
groEL
gene.
rpoH1
mutants are defective for growth at high temperature and form ineffective nodules,
rpoH1 rpoH2
double mutants are unable to form nodules, and
groESL1
mutants form ineffective nodules. To explore the roles of RpoH1 and RpoH2, we identified mutants that suppress both the growth and nodulation defects. These mutants do not suppress the nitrogen fixation defect. This implies that the functions of RpoH1 during growth and RpoH1/RpoH2 during the initiation of symbiosis are similar but that there is a different function of RpoH1 needed later during symbiosis. We showed that, unlike in
Escherichia coli
, overexpression of
groESL
is not sufficient to bypass any of the RpoH defects. Under free-living conditions, we determined that RpoH2 does not control expression of the
groE
genes, and RpoH1 only controls expression of
groESL5
. Finally, we completed the series of
groE
mutants by constructing
groESL3
and
groEL4
mutants and demonstrated that they do not display symbiotic defects. Therefore, the only
groESL
operon required by itself for symbiosis is
groESL1
. Taken together, these results suggest that GroEL/GroES production alone cannot explain the requirements for RpoH1 and RpoH2 in
S. meliloti
and that there must be other crucial targets.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
24 articles.
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