Analysis of transcriptionally active gene clusters of major outer membraneprotein multigene family in Ehrlichia canis and E-chaffeensis

Ehrlichia canis and E. chaffeensis are tick-borne obligatory intramonocytic ehrlichiae that cause febrile systemic illness in humans and dogs, respect ively. The current study analyzed the pleomorphic multigene family encoding approximately 30-kDa major outer membrane proteins (OMPs) of E. canis and E. chaffeensis. Upstream from secA and downstream of hypothetical transcrip tional regulator, 22 paralogs of the omp gene family were found to be tande mly arranged except for one or two genes with opposite orientations in a 28 - and a 27-kb locus in the E. canis and E. chaffeensis genomes, respectivel y. Each locus consisted of three highly repetitive regions with four nonrep etitive intervening regions. E. canis, in addition, had a 6.9-kb locus whic h contained a repeat of three tandem paralogs in the 28-kb locus. These tot al 47 paralogous and orthologous genes encoded OMPs of approximately 30 to 35 kDa consisting of several hypervariable regions alternating with conserv ed regions. In the 5'-end half of the 27-kb locus or the 28-kb locus of eac h Ehrlichia species, 14 paralogs were linked by short intergenic spaces ran ging from -8 bp (overlapped) to 27 bp, and 8 remaining paralogs in the 3'-e nd half were connected by longer intergenic spaces ranging from 213 to 632 bp. All 22 paralogs, five unknown genes, and secA in the omp cluster in E. canis were transcriptionally active in the monocyte culture, and the paralo gs with short intergenic spaces were cotranscribed with their adjacent gene s, including the respective intergenic spaces at both the 5' and the 3' sid es. Although omp genes are diverse, our results suggest that the gene organ ization of the clusters and the gene locus are conserved between two specie s of Ehrlichia to maintain a unique transcriptional mechanism for adaptatio n to environmental changes common to them.