EXOTIC PATHOGENS OF CITRUSTitle: Ca. Liberibacter asiaticus genes orthologous with pSymA-borne genes of Sinorhizobium meliloti: suggested roles in eukaryotic host interactions
Submitted to: American Phytopathological Society Potomac Division Meeting
Publication Type: Abstract Only
Publication Acceptance Date: March 14, 2012
Publication Date: June 1, 2012
Citation: Hartung, J.S., Shao, J.Y., Kuykendall, L.D. 2012. Ca. Liberibacter asiaticus genes orthologous with pSymA-borne genes of Sinorhizobium meliloti: suggested roles in eukaryotic host interactions. American Phytopathological Society Potomac Division Meeting. 102(6):S3-3.
‘Ca. Liberibacter asiaticus,’ is a psyllid-vectored, obligate phytopathogen associated with citrus huanglongbing disease. 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). ‘Ca. Liberibacter asiaticus’ has a reduced genome and we previously analyzed it for genes orthologous to genes on the S. meliloti circular chromosome. In this work we compare the gene content of the ‘Ca. Liberibacter asiaticus’ chromosome with that of the pSymA plasmid of S. meliloti. Most genes from ‘Ca. Liberibacter asiaticus’ with orthologs on pSymA also have orthologs on the S. meliloti 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’ genes were identified as having orthologous genes carried on pSymA without orthologs on the chromosome of S. meliloti. These two orthologous gene pairs encode a Na+/ K+ antiporter and an efflux protein for Co++, Zn++ and Cd++ cations. Also essential in maintaining homeostasis, a redox-regulated K+ efflux pump regulates cytoplasmic pH. The pSymA and ‘Ca. Liberibacter asiaticus’ orthologs of the latter protein are much more similar in amino acid alignment than are pSymA and S. meliloti chromosomal homologs. 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’ protein. These proteins are involved with amino acid uptake, cell surface modifications, chaperonins, electron transport, export of bioactive molecules, homeostasis, regulation of gene expression, signal transduction and synthesis of amino acids and metabolic cofactors. The presence of multiple orthologs on pSymA suggests that these proteins are of particular importance in host/microbe interaction and facilitates the ongoing evolution of these genes.