Spiroplasma citri is transmitted propagatively by the leafhopper, Circulifer tenellus. Movement of the wall-less prokaryote from the midgut to the salivary glands requires traversal of cellular barriers via adherence and endocytosis. Nontransmissible S. citri lines replicated in the hemocoel but did not cross the cellular barriers. Genome analysis revealed extensive chromosomal rearrangements between the transmissible and nontransmissible spiroplasma lines, including a large chromosomal inversion and deletions at each inversion border. One gene in the deleted region encodes a 58 kDA integral membrane protein having limited sequence similarity with adhesins of two zoopathogenic mycoplasmas. Adhesion of S. citri to a C. tenellus cell line was inhibited by proteinase K, and PAGE profiles of non-adherent, protease-treated spiroplasmas lacked a 89 kDa band. Adherence-deficient spiroplasmas, after 24-hr incubation in medium, re-gained both the 89 kDa protein (SARP1) and the ability to adhere to insect cells. These data suggest that adherence and transmissiblity are mediated by spiroplasma membrane protein(s). Anti-SARP1 monoclonal antibodies, used in western blotting and immunogold labeling, showed a surface location for this protein. The presence of SARP- and P58-like genes or proteins, determined by PCR, Southern and western blotting of spiroplasmas from several taxonomic groups, is not perfectly correlated with transmissibility. However, the SARP1 and P58 genes each are members of multi-gene families present in most Group I spiroplasmas. A model for insect transmission of phytopathogenic mollicutes is presented.
Keywords: Mollicute, Leafhopper transmission
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