The complete sequence of the 1,683-kb pSymB megaplasmid from the N
2
-fixing endosymbiont
Sinorhizobium melilotiJournal Articles
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abstract
Analysis of the 1,683,333-nt sequence of the pSymB megaplasmid from
the symbiotic N
2
-fixing bacterium
Sinorhizobium
meliloti
revealed that the replicon has a high gene density
with a total of 1,570 protein-coding regions, with few insertion
elements and regions duplicated elsewhere in the genome. The only
copies of an essential arg-tRNA gene and the
minCDE
genes are located on pSymB. Almost 20% of the pSymB sequence carries
genes encoding solute uptake systems, most of which were of the
ATP-binding cassette family. Many previously unsuspected genes involved
in polysaccharide biosynthesis were identified and these, together with
the two known distinct exopolysaccharide synthesis gene clusters, show
that 14% of the pSymB sequence is dedicated to polysaccharide
synthesis. Other recognizable gene clusters include many involved in
catabolic activities such as protocatechuate utilization and
phosphonate degradation. The functions of these genes are consistent
with the notion that pSymB plays a major role in the saprophytic
competence of the bacteria in the soil environment.
