Sequence and expression analysis of a surface antigen gene family of the rickettsia Anaplasma marginale

The tick-borne rickettsial organism, Anaplasma marginate, causes a disease in cattle of world-wide economic significance. This disease, anaplasmosis, is characterized by severe hemolytic anemia, high levels of rickettsemia an d, often, death in animals over 3 years of age. Animals that survive acute infection remain carriers, with continuous sub-microscopic cycles of ricket tsemia that can persist for the lifetime of the animal. In the search for p otential recombinant immunogens, it was discovered that several surface pro teins of A. film marginale encode polymorphic multigene families. Despite t he small size of the genome (approx. 1250 kb), these surface antigen gene f amilies comprise greater than 2% of the genome. We present here a mapping, sequencing and expression analysis of five complete or partial genes encodi ng MSP1b in a Florida strain of A. marginale. Two genes are complete; they encode mRNA that is translated into polypeptide products. Three genes are i ncomplete and appear to be derived from the complete genes by a series of s egmental intragenic recombinations. In two of the incomplete genes, 5' sequ ence in the incomplete genes is 3' sequence in the complete genes. Recombin ation within these gene families may generate diversity in surface antigens through combinatorial rearrangements. This could contribute to persistence in the chronic infections caused by A. marginale and related rickettsiae. (C) 2000 Elsevier Science B.V. All rights reserved.