Molecular analysis of a Chinese citrus tristeza virus isolate showing anomalous serological reactions

A Chinese isolate of Citrus tristeza virus (CTV), denoted HB1, was not recognized by a polyclonal antiserum (PAb-I) raised to the Italian CTV isolate 0002 and ten monoclonal antibodies to the Chinese isolate S4 (MAbs-S4). However, it reacted with a polyclonal antiserum (PAb-L5) raised to the Chinese isolate L5 in both ELISA and Western blots. To understand this unusual serological behaviour, the CP gene sequence of HB1 and other CTV isolates was determined. The results showed that HB1 was closely related to L5 at the nucleotide and amino acid levels. However, three amino acid residues in the HB1 CP differed from those of L5, which were conserved in all other isolates sequenced, including CTV-0002 and S4. This suggested that the failure of PAb-I and MAbs-S4 to recognise HB1 could depend on these molecular changes. Prediction of Antigenic Indexes and Surface Probability in four isolates (HB1, S4, L5 and 0002) with the Peptidestructure and Plotstructure softwares, showed that the HB1 tracing contained two small peaks at aa 84 and 190 that did not appear in any of the other profiles, but no apparent difference in aa 98. However, HB1 and L5 showed a similar small peak at the 5' end which did not occur in the S4 and CTV-0002.