Complete genome sequencing of Anaplasma marginale reveals that the surface is skewed to two superfamilies of outer membrane proteins

Abstract

The rickettsia Anaplasma marginale is the most prevalent tick-borne livestock pathogen worldwide and is a severe constraint to animal health. A. marginale establishes lifelong persistence in infected ruminants and these animals serve as a reservoir for ticks to acquire and transmit the pathogen. Within the mammalian host, A. marginale generates antigenic variants by changing a surface coat composed of numerous proteins. By sequencing and annotating the complete 1,197,687-bp genome of the St. Maries strain of A. marginale, we show that this surface coat is dominated by two families containing immunodominant proteins: the msp2 superfamily and the msp1 superfamily. Of the 949 annotated coding sequences, just 62 are predicted to be outer membrane proteins, and of these, 49 belong to one of these two superfamilies. The genome contains unusual functional pseudogenes that belong to the msp2 superfamily and play an integral role in surface coat antigenic variation, and are thus distinctly different from pseudogenes described as byproducts of reductive evolution in other Rickettsiales.rickettsiales ͉ bacterial artificial chromosome ͉ St. Maries strain

Superinfection as a driver of genomic diversification in antigenically variant pathogens

“…We first compared the diversity among the HVRs encoded by the full set of genomic msp2 donor alleles within each of five A. marginale strains, St. Maries, Florida, 6DE, B, and EM⌽ (12,13). Within strain HVR sequence diversity encoded by the donor alleles was similar ( Fig.…”

“…However, the presence of identical alleles at two different chromosomal loci within a strain (observed for both the St. Maries and Florida strains) (12) suggests that gene duplication may provide the template for additional interallelic recombination (22), alone or combined with mutation, to generate diverse alleles with consequent selection at the level of successful superinfection. The ability of a strain with a single diverse allele to superinfect long-term persistently infected hosts, represented by the experiments using the St. Maries and 6DE strains, support that duplication followed by divergence would confer a significant selective advantage.…”

“…The alleles were identified in the complete genome sequences of the St. Maries (GenBank accession no. CP000030) (12) and Florida strains (GenBank accession no. EU113268-75).…”

“…This mechanism is very similar to that used by African trypanosomes in which the complete (or nearly complete) variable surface glycoprotein (VSG) coding sequences or oligonucleotide segments are recombined into an expression site to generate a new VSG variant (11). Importantly, A. marginale has a relatively small genome (1/20th the size of Trypanosoma brucei), facilitating the sequencing of the complete repertoire of chromosomal msp2 donor alleles in each of multiple strains (9,12,13), data required to test the hypothesis but not yet available for other highly antigenically variable vector-borne pathogens such as Babesia, Plasmodium, and Trypanosoma (14 -17). In this article, we report the genomic-level allelic diversity among multiple strains of A. marginale, test whether allelic diversity between strains is linked to ability to infect an already persistently infected host, and propose a model in which host population immunity is a driving selective pressure for genomic diversification in highly antigenically variant pathogens.…”

Advances in the genomics of ticks and tick-borne pathogens

“…Examples include antigenic variation in Anaplasma marginale, which has long been the subject of active research. Completion of the A. marginale genome and the genomes of related organisms has enabled a comparative approach to this question [2]. Analysis of genome sequences from organisms as diverse as A. marginale, Borrelia hermsii and Trypanosoma brucei suggests convergent evolution to common mechanisms of antigenic variation and hence immune evasion (Palmer and Brayton, this issue).…”