ANAPLASMA-MARGINALE MAJOR SURFACE PROTEIN-3 IS ENCODED BY A POLYMORPHIC, MULTIGENE FAMILY

The immunodominant surface protein, MSP3, is structurally and antigeni cally polymorphic among strains of Anaplasma marginale. In this study we show that a polymorphic multigene family is at least partially resp onsible for the variation seen in MSP3. The A. marginale msp3 gene msp 3-12 was cloned and expressed in Escherichia coli. With msp3-12 as a p robe, multiple, partially homologous gene copies were identified in th e genomes of three A. marginale strains. These copies were widely dist ributed throughout the chromosome. Sequence analysis of three unique m sp3 genes, msp3-12, msp3-11, and msp3-19, revealed both conserved and variant regions within the open reading frames. Importantly, msp3 cont ains amino acid blocks related to another polymorphic multigene family product, MSP2. These data, in conjunction with data presented in prev ious studies, suggest that multigene families are used to vary importa nt antigenic surface proteins of A. marginale. These findings may prov ide a basis for studying antigenic variation of the organism in persis tently infected carrier cattle.