Sulsona C R, Mahan S M, Barbet A F
Department of Pathobiology, College of Veterinary Medicine, Gainesville, Florida, 32611, USA.
Biochem Biophys Res Commun. 1999 Apr 13;257(2):300-5. doi: 10.1006/bbrc.1999.0459.
Heartwater is an economically important disease of ruminants caused by the tick-transmitted rickettsia Cowdria ruminantium. The disease is present in Africa and the Caribbean and there is a risk of spread to the Americas, particularly because of a clinically asymptomatic carrier state in infected livestock and imported wild animals. The causative agent is closely related taxonomically to the human and animal pathogens Ehrlichia chaffeensis and Ehrlichia canis. A dominant immune response of infected animals or people is directed against variable outer membrane proteins of these agents known, in E. chaffeensis and E. canis, to be encoded by polymorphic multigene families. We demonstrate, by sequence analysis, that map1 encoding the major outer membrane protein of C. ruminantium is also encoded by a polymorphic multigene family. Two members of the gene family are located in tandem in the genome. The upstream member, orf2, is conserved, encoding only 2 amino acid substitutions among six different rickettsial strains from diverse locations in Africa and the Caribbean. In contrast, the downstream member, map1, contains variable and conserved regions between strains. Interestingly, orf2 is more closely related in sequence to omp1b of E. chaffeensis than to map1 of C. ruminantium. The regions that differ among orf2, map1, and omp1b correspond to previously identified variable sequences in outer membrane protein genes of E. chaffeensis and E. canis. These data suggest that diversity in these outer membrane proteins may arise by recombination among gene family members and offer a potential mechanism for persistence of infection in carrier animals.
心水病是一种由蜱传播的立克次氏体反刍兽考德里氏体引起的对反刍动物具有重要经济影响的疾病。该病存在于非洲和加勒比地区，并且有传播到美洲的风险，特别是因为受感染的家畜和进口野生动物存在临床无症状携带状态。病原体在分类学上与人类和动物病原体恰菲埃立克次氏体和犬埃立克次氏体密切相关。受感染动物或人的主要免疫反应针对这些病原体的可变外膜蛋白，在恰菲埃立克次氏体和犬埃立克次氏体中，这些蛋白由多态性多基因家族编码。我们通过序列分析证明，编码反刍兽考德里氏体主要外膜蛋白的map1也由一个多态性多基因家族编码。该基因家族的两个成员在基因组中串联排列。上游成员orf2是保守的，在来自非洲和加勒比地区不同地点的六种不同立克次氏体菌株中仅编码两个氨基酸替换。相比之下，下游成员map1在菌株之间包含可变区和保守区。有趣的是，orf2在序列上与恰菲埃立克次氏体的omp1b比与反刍兽考德里氏体的map1更密切相关。orf2、map1和omp1b之间不同的区域对应于恰菲埃立克次氏体和犬埃立克次氏体外膜蛋白基因中先前确定的可变序列。这些数据表明，这些外膜蛋白的多样性可能通过基因家族成员之间的重组产生，并为携带动物中感染的持续存在提供了一种潜在机制。
