Proteomic Identification of a Novel Anaplasma phagocytophilum DNA Binding Protein That Regulates a Putative Transcription Factor

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Abstract
Anaplasma phagocytophilum , the etiologic agent of human granulocytic anaplasmosis, is an obligatory intracellular bacterium. Little is known about the gene regulatory mechanisms for this bacterium. A gene encoding a putative transcription factor, tr1 , upstream of three tandem genes encoding outer membrane proteins, including the major outer membrane protein P44, is driven by a strong promoter. In the present study, gel mobility shift assays revealed the presence of A. phagocytophilum proteins that interact with the promoter region of tr1 . These proteins interacting with the tr1 promoter region were purified by biotin-labeled DNA affinity chromatography from a large amount of host cell-free bacteria. Mass spectrometry identified the major protein as an A. phagocytophilum 12.5-kDa hypothetical protein, which was named ApxR. In a DNase I protection assay, recombinant ApxR (rApxR) bound cooperatively to four 24- or 25-bp sites within 235 bp upstream of tr1 : regions III and IV proximal to tr1 had higher affinity than regions I and II did. Deletion assays showed that regions III and IV were essential for rApxR binding, whereas regions I and II upstream of regions III and IV were not. The primary cis -acting region was region IV, since region IV alone was sufficient for rApxR to strongly transactivate the downstream gene in a lacZ reporter assay. Addition of regions I, II, and III did not enhance transactivation. These results show that ApxR is a novel transcriptional regulator that directly regulates tr1 .