Abstract
An intracellular plant pathogen ‘Candidatus Liberibacter asiaticus,’ a member of the Rhizobiales, is related to Sinorhizobium meliloti, Bradyrhizobium japonicum, nitrogen fixing endosymbionts, Agrobacterium tumefaciens, a plant pathogen, and Bartonella henselae, an intracellular mammalian pathogen. Whole chromosome comparisons identified at least 50 clusters of conserved orthologous genes found on the chromosomes of all five metabolically diverse species. The intracellular pathogens ‘Ca. Liberibacter asiaticus’ and Bartonella henselae have genomes drastically reduced in gene content and size as well as a relatively low content of guanine and cytosine. Codon and amino acid preferences that emphasize low guanosine and cytosine usage are globally employed in these genomes, including within regions of microsynteny and within signature sequences of orthologous proteins. The length of orthologous proteins is generally conserved, but not their isoelectric points, consistent with extensive amino acid substitutions to accommodate selection for low GC content. The ‘Ca. Liberibacter asiaticus’ genome apparently has all of the genes required for DNA replication present in Sinorhizobium meliloti except it has only two, rather than three RNaseH genes. The gene set required for DNA repair has only one rather than ten DNA ligases found in Sinorhizobium meliloti, and the DNA PolI of ‘Ca. Liberibacter asiaticus’ lacks domains needed for excision repair. Thus the ability of ‘Ca. Liberibacter asiaticus’ to repair mutations in its genome may be impaired. Both ‘Ca. Liberibacter asiaticus and Bartonella henselae lack enzymes needed for the metabolism of purines and pyrimidines, which must therefore be obtained from the host. The ‘Ca. Liberibacter asiaticus’ genome also has a greatly reduced set of sigma factors used to control transcription, and lacks sigma factors 24, 28 and 38. The ‘Ca. Liberibacter asiaticus’ genome has all of the hallmarks of a reduced genome of a pathogen adapted to an intracellular lifestyle.
Highlights
Huanglongbing (HLB), arguably the most serious disease of citrus worldwide [1,2], originated in South Asia [3,4] and recently became epidemic in Brazil [5] and in Florida [6] homes of the largest orange juice industries in the world
Isoelectric points and the number of amino acids predicted for orthologous protein pairs were plotted for ‘Ca. Liberibacter asiaticus’ vs S. meliloti, B. japonicum, A. tumefaciens, and B. henselae
The mean pI for 552 orthologous proteins for ‘Ca. Liberibacter asiaticus vs S. meliloti was 7.90 vs 6.91
Summary
Huanglongbing (HLB), arguably the most serious disease of citrus worldwide [1,2], originated in South Asia [3,4] and recently became epidemic in Brazil [5] and in Florida [6] homes of the largest orange juice industries in the world. Isoelectric points (pIs) and the number of amino acids predicted for orthologous protein pairs were plotted for ‘Ca. Liberibacter asiaticus’ vs S. meliloti, B. japonicum, A. tumefaciens, and B. henselae.
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