The stolbur phytoplasma antigenic membrane protein gene stamp is submitted to diversifying positive selection

Surface proteins play an important role in phytoplasma life cycle. The antigenic membrane protein (AMP) of 'Candidatus Phytoplasma asteris' has been shown to interact with the insect microfilaments. Due to the synteny of the groL-amp-nadE genetic locus between phytoplasma genomes, the gene stamp that encodes the antigenic membrane protein of stolbur phytoplasma has been cloned and characterized. It encodes a 157 amino acid-long protein with a predicted signal peptide and a C-terminal hydrophobic alpha-helix. STAMP was 26-40% identical to AMP of 'Candidatus Phytoplasma asteris' strains and 40% identical to AMP of 'Ca. P. japonicum'. The expression of STAMP in Escherichia. coli produced a 16 kDa peptide recognized by an anti-stolbur monoclonal antibody. Stamp was more variable than the house-keeping gene secY and the ratio between non-synonymous over synonymous mutations (dN/dS ) was 2.78 for stamp as compared to 0.64 for secY. This indicates that stamp is submitted to a positive diversifying selection pressure.