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United States Department of Agriculture

Agricultural Research Service


item Futse, James
item Brayton, Kelly
item Knowles, Donald - Don
item Palmer, Guy

Submitted to: Molecular Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2005
Publication Date: 7/5/2005
Citation: Futse, J.E., Brayton, K.A., Knowles Jr, D.P., Palmer, G.H. 2005. Structural basis for segmental gene conversion in generation of anaplasma marginale outer membrane protein variants. Molecular Microbiology. 57(1):212-221.

Interpretive Summary: An important component in the development of effective vaccines for organisms that survive for life in the host is understanding how such organisms persist. In this study we examined one of the components of Anaplasma marginale which helps the organism persist in an infected cow for the life of the cow. Based on the collective data we proposed a mechanism by which this particular component of A. marginale is able to change and help the organism persist for the life of the cow. A. marginale causes a severe loss of red blood cells during the initial phase of infection and if the cow survives the organism then remains within the cow for life.

Technical Abstract: Bacterial pathogens in the genus Anaplasma generate surface coat variants by gene conversion of chromosomal pseudogenes into single-expression sites. These pseudogenes encode unique surface-exposed hypervariable regions flanked by conserved domains, which are identical to the expression site flanking domains. In addition, Anaplasma marginale generates variants by recombination of oligonucleotide segments derived from the pseudogenes into the existing expression site copy, resulting in a combinatorial increase in variant diversity. Using the A. marginale genome sequence to track the origin of sequences recombined into the msp2 expression site, we demonstrated that the complexity of the expressed msp2 increases during infection, reflecting a shift from recombination of the complete hypervariable region of a given pseudogene to complex mosaics with segments derived from hypervariable regions of different pseudogenes. Examination of the complete set of 1183 variants with segmental changes revealed that 99% could be explained by one of the recombination sites occurring in the conserved flanking domains and the other within the hypervariable region. Consequently, we propose an 'anchoring' model for segmental gene conversion whereby the conserved flanking sequences tightly align and anchor the expression site sequence to the pseudogene. Associated with the recombination sites were deletions, insertions and substitutions; however, these are a relatively minor contribution to variant generation as these occurred in less than 2% of the variants. Importantly, the anchoring model, which can account for more variants than a strict segmental sequence identity mechanism, is consistent with the number of msp2 variants predicted and empirically identified during persistent infection.

Last Modified: 05/25/2017
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