Genetic diversity of the 28-kilodalton outer membrane protein gene in human isolates of Ehrlichia chaffeensis

The Ehrlichia chaffeensis 28-kDa outer membrane protein (p28) gene was sequ enced completely by genomic walking with adapter PCR, The DNA sequence of t he p28 gene was nearly identical to the previously reported sequence (N. Oh ashi, N, Zhi, Y, Zhang, and Y, Rikihisa, Infect. Immun, 66:132-139, 1998), but analysis of a further 75 bp on the 5' end of the gene revealed DNA that encoded a 25-amino-acid signal sequence, The leader sequence was removed f rom the N terminus of a 30-kDa precursor to generate the mature p28 protein . A monoclonal antibody (MAb), 1A9, recognizing four outer membrane protein s of E. chaffeensis (Arkansas strain) including the 25-, 26-, 27-, and 29-k Da proteins (X,-J, Yu, P, Brouqui, J, S, Dumler, and D, Raoult, J, Clin, Mi crobiol, 31:3284-3288, 1993) reacted with the recombinant p28 protein. This result indicated that the four proteins recognized by MAb 1A9 were encoded by the multiple genes of the 28-kDa protein family. DNA sequence alignment analysis revealed divergence of p28 among all five human isolates of E, ch affeensis, The E. chaffeensis strains could be divided into three genetic g roups on the basis of the p28 gene. The first group consisted of the Sapulp a and St. Vincent strains. They had predicted amino acid sequences identica l to each other. The second group contained strain 91HE17 and strain Jar, w hich only showed 0.4% divergence from each other, The third group contained the Arkansas strain only. The amino acid sequences of p28 differed by 11% between the first two groups, by 13.3% between the first and third groups, and by 13.1% between the second and third groups. The presence of antigenic variants of p28 among the strains of E, chaffeensis and the presence of mu ltiple copies of heterogeneous genes suggest a possible mechanism by which E, chaffeensis might evade the host immune defenses, Whether or not immuniz ation with the p28 of one strain of E. chaffeensis would confer cross-prote ction against other strains needs to be investigated.