Genomic Cloning and Sequence Analysis of Trypanosoma brucei rhodesiense gene Encoding Putative N-glycosylation Enzyme

Abstract

Background: Trypanosoma brucei rhodesiense is a haemoflagellate parasite of zoonotic significance. Aside from its public health importance, this parasite subspecies gained notoriety because of their effective system to circumvent the immune response of vertebrate host. The parasite cell surface is covered with millions of VSG dimers, which serve as an almost infinite repertoire of biomolecules needed for evasion of host immune system. Around two decades ago, it was resolved that all trypanosome VSG is associated with one or more N-linked oligosaccharides, with a range of structures including high mannose and complex types. This complex process of protein modification known as N-linked glycosylation is catalyzed by oligosaccharyl transferase (OST). In general, the incorporation of glycan structures can alter protein’s antigenic properties and recently it was established that glycan molecules associated with VSG were found to be important in several aspects of trypanosome-host interaction, especially during parasite evasion of the host defense mechanisms. Therefore, our major interest is to clone and characterize the trypanosome OST. Materials, Methods & Results: The template genomic DNA for PCR amplification was extracted as described previously. In an attempt to clone Trypanosoma brucei rhodesiense putative oligosaccharyl transferase, an amplicon of ~2000 bp was obtained having an open reading frame of 2057 bp and deduced primary structure composed of 685 amino acid residues (TbrOST II). Comparison of TbrOST II ORF with annotated putative oligosaccharyl transferase in the genome of other organisms revealed sequence identity to other kinetoplastid. TbrOST II had high nucleotide (Ns) and amino acid (As) sequence similarity with the genomes of T. brucei gambiense (Ns:99%; As:78%) and T. brucei (Ns:95-98%; As:77%-98%). There was also significant nucleotide and amino acid sequence identity in the genomes of T. cruzi (Ns:74%; As:63%), Leishmania infantum (Ns:70-83%; As:46-57%), L. braziliensis (Ns:69-81%; As:46-55%) and L. major (Ns:69-80%; As:46-57%). Sequence similarity (71-77%) from other origins was also exhibited. The nucleotide sequence alignments and analysis were performed using the Oxford University Mac Vector 6.5 sequence analysis software and CLC Workbench 5.6 software. Discussion: The nucleotide BLAST results indicate that sequence identity is higher between species of the same genus rather than of the same family. It is known that T. brucei, T. gambiense and T. rhodesiense are members of the Brucei-complex or Brucei group. Although T. brucei brucei has more similarities with T. brucei rhodesiense than T. brucei gambiense, these parasites are morphologically indistinguishable. This is the probable reason why high sequence identity was displayed by other subspecies of the Brucei group. In addition, the high percent identity possessed by TbrOST II with other trypansomatids agrees with the evolutionarily conserved characteristics of the established OST. The DNA sequence data of TbrOST II showing similar sequences in the genome of other organisms further corroborate the previous reports regarding the ubiquitous nature of OST in other life forms. Based on the size of the amplicon and significant percentage of nucleotide and amino acid sequence identity to homologues within the genome of related species and various organisms, the results strongly indicate that TbrOST II is a trypanosome oligosaccharyl transferase gene candidate that should be fully characterized and subjected to functional genomic studies. The study reports the molecular cloning and sequencing of a potential oligosaccharyl transferase gene in T. brucei rhodesiense (TbrOST II). The sequence data has been deposited in the GenBank with accession number GU475126.