Phytoplasmas are intracellular bacterial pathogens of the class Mollicutes that can replicate in both plants and insects. The genome of Aster yellows phytoplasma strain witches’ broom (AY-WB) is predicted to encode 56 secreted virulence (effector) proteins, and we have determined that the expression of most of these genes is host-specific. However, the mechanism(s) by which phytoplasma gene expression is regulated is entirely unknown. Furthermore, only one of fifty-six putative effectors has been characterized.In this work, we seek to identify the regulatory elements that specify a host-specific transcriptional response by mapping the promoters of genes in insect- and plant-colonizing AY-WB. A comparison of plant-specific promoters versus those expressed primarily in insects will afford an opportunity to identify determinants important to the regulation of phytoplasma genes in these diverse hosts. Furthermore, we identify six candidate effector proteins which we hypothesize are secreted by AY-WB upon infection of a plant. We will determine if these proteins are genuine phytoplasma effectors by assessing their ability to interact with a plant and modulate plant phenotype. We will also employ immunohistology to determine whether these proteins migrate from the phloem to target developing tissues. As a means of identifying interacting proteins, we will perform a yeast two-hybrid screen against an Arabidopsis library, an initiative which has proven successful in the study of another AY-WB effector protein, SAP11. The proposed research project will offer valuable insight into the mechanisms by which phytoplasmas infect plants and modulate host growth and development.