UC Riverside

Gram negative pathogens rely on the Type III secretion system to inject virulence proteins (aka secrete effectors) into eukaryotic hosts to manipulate defense. YopJ is one of the largest effector family prevalently produced by both animal and plant pathogens. HopZ1a from the plant pathogen Pseudomonas syringae pv. syringae A2 possesses an acetyltransferase activity and acetylates target proteins in plant hosts for virulence.

In chapter I, I used biochemical approaches to characterize the residues required for the enzymatic activity of HopZ1a. I identified a threonine residue T346 as the autoacetylation site of HopZ1a. I also found two serine residues S349 and S351 required for the avirulence and virulence function of HopZ1a, through their contributions to the acetyltransferase activity. The acetyltransferase activity of HopZ1a can be activated by the eukaryotic cofactor inositol hexakisphosphate (IP6). My experiments showed that S349 and S351, together with a previously identified lysine residue K289, were essential to mediate IP6-induced conformational change of HopZ1a. The conservation of S349 and S351 and the requirement of them in the acetyltransferase activity of other YopJ family effectors suggest that additional residues in addition to the indispensable catalytic triad are required for the biological function of HopZ1a.

In chapter II, I used traditional genetics and molecular biology approaches to study the function of an interacting partner of HopZ1a in soybean called GmZINP2. By using transgenic soybeans over-expressing GmZINP2, I showed that GmZINIP2 is a negative regulator of plant immunity and suppresses early and late defense responses. GmZINP2 contains domains that are likely associated with chromatin remodeling functions. I showed that GmZINP2 could bind DNA and also interacts with a transcription factor GmMYB56 in soybean. Interestingly, HopZ1a could acetylate GmZINP2 and affect the interaction between GmZINP2 and GmMYB56. As such, HopZ1a may manipulate soybean defense through its interaction with GmZINP2.

This thesis represents a comprehensive study on the enzymatic activity and virulence functions of an important class of type III effectors. The study on GmZINP2 revealed possible manipulation of host defense by bacterial pathogens through chromatin remodeling process, which is a novel strategy utilized by bacterial pathogens to enhance virulence.