Identification of a novel 23 kDa protein encoded by putative open reading frame 2 of TT virus (TTV) genotype 1 different from the other genotypes

We report the entire open reading frames (ORFs) sequences of four TT virus (TTV) isolates, one genotype 2 (G2) and three G4 isolates. Despite a DNA vi rus, TTV possesses high rate of amino acid (aa) substitution: the aa sequen ce homology of ORF1 and 2 is lower than the nucleotide homology. The partia l 'N22' region of ORF1 is suitable for genotyping of 'prototype TTV' isolat es, because the phylogenetic tree from partial 'N22' sequence is consistent with that from the entire ORF1. Based on our sequence data, ORF2 from most isolates excluding G1 encode truncated 49 aa (pORF2a) because of an in-fra me stop codon, although ORF2s from most GI isolates encode 202 aa (pORF2ab) . Just downstream the stop codon, another ORF encoding a protein of approxi mately 150 aa (pORF2b) is found, whose homology is quite low among these ge notypes. Our in vitro transcription/translation study supports that all Gla and a part of Gib without an in-frame stop codon dominantly encode pORF2ab , a novel 23 kDa protein, whereas the other genotypes with an in-frame stop codon encode pORF2b (17 kDa). Our data indicate TTV G1a and a part of G1b should have different characteristics from the other genotypes.