Abstract
Sinorhizobium meliloti strain 1021, a nitrogen-fixing, root-nodulating bacterial microsymbiont of alfalfa, has a 3.5 Mbp circular chromosome and two megaplasmids including 1.3 Mbp pSymA carrying nonessential ‘accessory’ genes for nitrogen fixation (nif), nodulation and host specificity (nod). A related bacterium, psyllid-vectored ‘Ca. Liberibacter asiaticus,’ is an obligate phytopathogen with a reduced genome that was previously analyzed for genes orthologous to genes on the S. meliloti circular chromosome. In general, proteins encoded by pSymA genes are more similar in sequence alignment to those encoded by S. meliloti chromosomal orthologs than to orthologous proteins encoded by genes carried on the ‘Ca. Liberibacter asiaticus’ genome. Only two ‘Ca. Liberibacter asiaticus’ proteins were identified as having orthologous proteins encoded on pSymA but not also encoded on the chromosome of S. meliloti. These two orthologous gene pairs encode a Na+/K+ antiporter (shared with intracellular pathogens of the family Bartonellacea) and a Co++, Zn++ and Cd++ cation efflux protein that is shared with the phytopathogen Agrobacterium. Another shared protein, a redox-regulated K+ efflux pump may regulate cytoplasmic pH and homeostasis. The pSymA and ‘Ca. Liberibacter asiaticus’ orthologs of the latter protein are more highly similar in amino acid alignment compared with the alignment of the pSymA-encoded protein with its S. meliloti chromosomal homolog. About 182 pSymA encoded proteins have sequence similarity (≤E-10) with ‘Ca. Liberibacter asiaticus’ proteins, often present as multiple orthologs of single ‘Ca. Liberibacter asiaticus’ proteins. These proteins are involved with amino acid uptake, cell surface structure, chaperonins, electron transport, export of bioactive molecules, cellular homeostasis, regulation of gene expression, signal transduction and synthesis of amino acids and metabolic cofactors. The presence of multiple orthologs defies mutational analysis and is consistent with the hypothesis that these proteins may be of particular importance in host/microbe interaction and their duplication likely facilitates their ongoing evolution.
Highlights
Within the order Rhizobiales of the class alpha-Proteobacteria, Sinorhizobium meliloti is a member of a diverse bacterial family, Rhizobiaceae [1–3]
Proteins encoded by pSymA genes are more similar in sequence alignment to those encoded by S. meliloti chromosomal orthologs than to orthologous proteins encoded by genes carried on the ‘Ca. Liberibacter asiaticus’ genome
Maintenance of Homeostasis In general, proteins encoded by pSymA genes are more similar in sequence alignment to those encoded by S. meliloti chromosomal orthologs than to orthologous proteins encoded by genes carried on the ‘Ca. Liberibacter asiaticus’ genome
Summary
Within the order Rhizobiales of the class alpha-Proteobacteria, Sinorhizobium meliloti is a member of a diverse bacterial family, Rhizobiaceae [1–3]. A growing number of species of the family Rhizobiaceae and of the order Rhizobiales are becoming recognized based on 16 S RNA gene sequence data. ‘Ca. Liberibacter asiaticus’ is recognized as the causal agent of huanglongbing (HLB), known as citrus greening disease. This phytopathogen was introduced into the New World from Asia [5,6], and is transmitted by the citrus psyllid, Diaphorina citri [7]. The full genomic sequence of ‘Ca. Liberibacter asiaticus’ was determined by deep sequencing of total DNA obtained from a single citrus psyllid which contained more than 108 cells of ‘Ca. Liberibacter asiaticus.’.
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