RESUMO
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.
Assuntos
Plasmídeos/genética , Sinorhizobium meliloti/genética , Proteínas de Bactérias/genética , Proteínas de Bactérias/fisiologia , Metabolismo dos Carboidratos , Proteínas de Transporte/genética , Chaperoninas/genética , DNA Bacteriano/genética , Resistência Microbiana a Medicamentos/genética , Enzimas/genética , Regulação Bacteriana da Expressão Gênica , Biblioteca Gênica , Genes Bacterianos , Lipopolissacarídeos/biossíntese , Dados de Sequência Molecular , Fixação de Nitrogênio/genética , RNA de Transferência de Arginina/genética , Origem de Replicação/genética , Replicon/genética , Análise de Sequência de DNA , Especificidade da Espécie , Transcrição Gênica/genéticaRESUMO
The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N2) to ammonium (NH4+). We present here the annotated DNA sequence of the alpha-proteobacterium Sinorhizobium meliloti, the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.
