By Alan G . Barbour , * Osmar Barrera , and Ralph

Borrelia hermsii, a spirochete and an agent of relapsing fever, undergoes spontaneous antigenic variation both in vivo (1, 2) and in vitro (2). The rate of serotypic change is estimated to be 10-3-10 -4 per cell per generation (2). The antigenic repertoire of B. hermsii (2) may be as extensive as that of another vector-transmitted and blood-borne pathogen, Trypanosoma brucei, a eucaryote (3, 4). Examination of four variants or serotypes found among the progeny of a single B. hermsii organism revealed that each serotype had an abundant protein of unique apparent molecular weight (5). Immunofluorescence and radioiodinelabeling studies indicated that these variable proteins, the pI proteins, were located on the surface. Serotype-specific monoclonal antibodies bound to homologous but not heterologous pI proteins (5). The pII protein was the other major protein found in whole cell lysate of each serotype (5). In contrast to pI proteins, the pII proteins had the same apparent molecular weight in each serotype, were less accessible than the pI proteins for iodine labeling, and were recognized in western blots by heterologous as well as homologous polyclonal antiserotype sera (5). As it appears likely that antigenic variation is based on the spontaneous appearance in a borrellia population of an organism possessing an antigenically "novel" pI protein, we are studying the structure of the pI proteins. Of particular importance for models of the genetic mechanism of antigenic variation is the amount of structural relatedness among the different pI proteins. We analyzed the pI and pII proteins of serotypes C, 7, and 21 with oneand two-dimensional (l-D, 2-D) I peptide-mapping procedures. The results indicated that whereas all pII proteins are identical, the pI proteins have little, if any, apparent sequence homology among them.