Affiliation:
1. Center for Environmental Genomics, Department of Biology, McMaster University, Hamilton, Ontario, Canada
Abstract
ABSTRACT
Toxin and antitoxin (TA) gene pairs are addiction systems that are present in many microbial genomes.
Sinorhizobium meliloti
is an N
2
-fixing bacterial symbiont of alfalfa and other leguminous plants, and its genome consists of three large replicons, a circular chromosome (3.7 Mb) and the megaplasmids pSymA (1.4 Mb) and pSymB (1.7 Mb).
S. meliloti
carries 211 predicted type II TA genes, each encoding a toxin or an antitoxin. We constructed defined deletion strains that collectively removed the entire pSymA and pSymB megaplasmids except for their
oriV
regions. Of approximately 100 TA genes on pSymA and pSymB, we identified four whose loss was associated with cell death or stasis unless copies of the genes were supplied in
trans
. Orthologs of three of these loci have been characterized in other organisms (
relB/E
[
sma0471/sma0473
],
Fic
[
DOC
] [
sma2105
], and
VapC
[
PIN
] [
orf2230/sma2231
]), and this report contains the first experimental proof that
RES/Xre
(
smb21127/smb21128
) loci can function as a TA system. Transcriptome sequencing (RNA-seq) analysis did not reveal transcriptional differences between the TA systems to account for why deletion of the four “active” systems resulted in cell toxicity. These data suggest that severe cell growth phenotypes result from the loss of a few TA systems and that loss of most TA systems may result in more subtle phenotypes. These four TA systems do not appear to play a direct role in the
S. meliloti
-alfalfa symbiosis, as strains lacking these TA systems had a symbiotic N
2
fixation phenotype that was indistinguishable from the wild type.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
42 articles.
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