Comparative characterization of two membrane proteins in phytoplasmas of the group 16SrII

Phytoplasmas are known as major plant pathogens. In this study two membrane proteins of lime witches’ broom, alfalfa witches’ broom (strains Fars and Systan) and cucumber phyllody phytoplasmas were characterized. The Immunodominant membrane protein (Imp) was amplified and sequenced from all 4 phytoplasma strains. Analysis showed that the Imp protein had a transmembrane domain near the N-terminus region. 20 amino acids of the N-terminal region were predicted to be inside the cytoplasm and the major part of the protein at C-terminal (130 aa) located in extracytoplasmic region. Imp sequence identity among the phytoplasmas under study ranged between 79 and 99%. The dN/dS value calculated on the Imp was 0.8 (negative selection). However, several positively selected amino acids were found on this gene. The sequence identity of another membrane protein, IntImp, among these phytoplasmas ranged between 85 and 100%. The deduced amino acid sequence of the Intimp was predicted to include 6 transmembrane domains with an N-terminal in, C-terminal out topology. The dN/dS value on the IntImp was calculated to be <1 (0.5), so that the gene is under strong negative selection pressure. No positively selected amino acid sites was found along the IntImp gene. Due to the higher variability and the presence of positively selected sites, the Imp has the higher potential for differentiation of close phytoplasmas and also for interaction with host cell proteins.

Keywords:

phytoplasma , protein , membrane , selection pressure