Introduction

TAL Effectors (which stands for Transcription Activator-Like Effectors) are a family of proteins that bind DNA with a high sequence specificity and interfere with cellular activities by causing the expression of specific genes.

Also known as AvrBs3 family effectors (Boch J. et al, 2010), they were initially discovered in a pathogenic Proteobacteria called Xanthomonas which inject the effector proteins into plant cells causing several diseases in crop species (Kay, Hahn et al. 2007).

Figure 1.1 Wheat with Blight disease caused by Xanthomonas

(Image courtesy of Virginia Polytechnic Institute and State University)

The secreted TAL effectors (TALE’s) are injected into the plants via the Type III Secretion System, which is one of the five bacterial secretion systems found in Gram- negative bacteria (Gerlach, Hensel 2007).

The Type III Secretion System apparatus is a needle-like appendage consisting of a base section embedded in the bacterial inner cell wall with a connector domain between it and the outer membrane anchoring unit. The needle, which is the extracellular secreting domain, penetrates the eukaryotic cell membrane and injects the effector proteins, including TAL effectors, straight from the bacterial cytoplasm into the eukaryotic host cell (Bing Yang et. al, 2006)

Figure 1.2 EM construct Type III Secretion System Needle

(Image courtesy of Lea Group, University of Oxford)

The TALE’s are then imported into the nucleus of the cell where they target specific genes due to precise structural features recognising explicit DNA sequences. In plants, the TALE’s, once bound to the DNA, cause the production of proteins that aid in bacterial infection of the eukaryotic host cell (Bogdanove, Schornack et al. 2010).