Analysis of complete genome sequence of Neorickettsia risticii: causative agent of Potomac horse fever

Abstract

Neorickettsia risticii is an obligate intracellular bacterium of the trematodes and mammals. Horses develop Potomac horse fever (PHF) when they ingest aquatic insects containing encysted N. risticii-infected trematodes. The complete genome sequence of N. risticii Illinois consists of a single circular chromosome of 879 977 bp and encodes 38 RNA species and 898 proteins. Although N. risticii has limited ability to synthesize amino acids and lacks many metabolic pathways, it is capable of making major vitamins, cofactors and nucleotides. Comparison with its closely related human pathogen N. sennetsu showed that 758 (88.2%) of protein-coding genes are conserved between N. risticii and N. sennetsu. Four-way comparison of genes among N. risticii and other Anaplasmataceae showed that most genes are either shared among Anaplasmataceae (525 orthologs that generally associated with housekeeping functions), or specific to each genome (>200 genes that are mostly hypothetical proteins). Genes potentially involved in the pathogenesis of N. risticii were identified, including those encoding putative outer membrane proteins, two-component systems and a type IV secretion system (T4SS). The bipolar localization of T4SS pilus protein VirB2 on the bacterial surface was demonstrated for the first time in obligate intracellular bacteria. These data provide insights toward genomic potential of N. risticii and intracellular parasitism, and facilitate our understanding of PHF pathogenesis.

Proteomic Analysis of Neorickettsia sennetsu Surface-Exposed Proteins and Porin Activity of the Major Surface Protein P51

“…P51 and Nsp3, found so far only in Neorickettsia spp., had the largest numbers of identified peptides (138 and 95, respectively), and Nsp3 and Nsp2 (another unique protein of Neo- Proteomic analysis further characterized unique and hypothetical Neorickettsia proteins. This is the first time that NSE_0908, known as strain-specific antigen 3 (Ssa3) (29), was found to be expressed. Ssa proteins of N. risticii were previously proposed to be a potential cause of vaccine failure (5).…”

“…The secondary structure of N. sennetsu P51 was predicted using a combination of the programming algorithm in the PRED-TMBB web server (1), the hydrophobicity and hydrophobic movement profile (23), and DNAStar MegAlign (DNAStar, Madison, WI) alignment and analysis of the P51 sequences of the N. sennetsu type strain (20), Neorickettsia risticii strain Illinois (type strain) (29), and the Stellantchasmus falcatus agent (SF agent) Hirose strain (44).…”

“…Ssa proteins of N. risticii were previously proposed to be a potential cause of vaccine failure (5). One of three N. sennetsu Ssa paralogs, Ssa3 (29), was found to be surface exposed, although no signal peptide for secretion was predicted for Ssa3 or any other Ssa (SignalP, v.3.0). A strongly identified protein, NSE_0456, is conserved in Neorickettsia (20,29), has no homology to other bacterial proteins, and has an unknown function.…”

“…One of three N. sennetsu Ssa paralogs, Ssa3 (29), was found to be surface exposed, although no signal peptide for secretion was predicted for Ssa3 or any other Ssa (SignalP, v.3.0). A strongly identified protein, NSE_0456, is conserved in Neorickettsia (20,29), has no homology to other bacterial proteins, and has an unknown function. Other hypothetical proteins that were identified by proteomics, conserved in Neorickettsia, and not found in other bacteria include NSE_0687, NSE_0591 (no BLASTP results other than that for N. risticii), and NSE_0732.…”

“…P44/Msp2 and OMP-1/P28/P30 proteins belong to the family pfam01617, and the N. sennetsu genome was reported to encode only one hypothetical protein from this family (GenBank accession no. NSE_0875) (20), later named Neorickettsia surface protein 3 (Nsp3) (29).…”