The map1 gene of Cowdria ruminantium is a member of a multigene family containing both conserved and variable genes

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, par ticularly because of a clinically asymptomatic carrier state in infected li vestock and imported wild animals. The causative agent is closely related t axonomically to the human and animal pathogens Ehrlichia chaffeensis and Eh rlichia 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 famili es. We demonstrate, by sequence analysis, that map1 encoding the major oute r membrane protein of C. ruminantium is also encoded by a polymorphic multi gene family. Two members of the gene family are located in tandem in the ge nome. The upstream member, orf2, is conserved, encoding only 2 amino acid s ubstitutions among six different rickettsial strains from diverse locations in Africa and the Caribbean. In contrast, the downstream member, map1, con tains variable and conserved regions between strains. Interestingly, orf2 i s 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 corr espond to previously identified variable sequences in outer membrane protei n genes of E. chaffeensis and E. canis. These data suggest that diversity i n these outer membrane proteins may arise by recombination among gene famil y members and offer a potential mechanism for persistence of infection in c arrier animals. (C) 1999 Academic Press.