Virulence of 'Candidatus Phytoplasma mali' strains is closely linked to conserved substitutions in AAA+ ATPase AP460 and their supposed effect on enzyme function

In continuing previous work on the role of AAA+ proteins of the apple proliferation agent 'Candidatus Phytoplasma mali' in phytoplasma virulence and suppression of virulence, 147 full-length deduced protein sequences of AAA+ ATPase AP460 of single- and multiple-strain accessions were examined. This approach revealed that in this protein two regions can be distinguished. Region 1, located in the N-terminal part, is characterized by 14 highly conserved substitutions associated with the suppression of virulence. However, these substitutions were not present in all attenuated strains. In the more diverse region 2, located in the C-terminal part, highly conserved substitutions associated with two groups of virulence were identified. In addition to the virulence-related residues, three other groups of conserved substitutions are present in strains with attenuated virulence with or without the presence of suppression-associated substitutions in region 1. In one of these groups, substitutions next to key residues of the ATPase motifs sensor 1 and 2 and arginine finger do occur that seem to affect ATPase function. This group of substitution was present in all attenuated strains. From these findings it can be concluded that two different mechanisms of suppression exist of which the supposed effect on ATPase function seems to be more important than the suppression-associated substitutions in region 1. The presence of virulent, avirulent and suppressive strains in a tree leads in the commonly occurring multiple infections to interactions in which the resulting virulence is determined by the numerical relation of virulent and attenuated strains.