Trna Sintetasa

BREVIA
Major Biocontrol of Plant Tumors Targets tRNA Synthetase
John S. Reader,1 Phillip T. Ordoukhanian,1 Jung-Gun Kim,2 ´ Valerie de Crecy-Lagard,1 Ingyu Hwang,2 Stephen Farrand,3 Paul Schimmel1*Infection of plants by pathogenic strains of Agrobacterium tumefaciens causes crown gall tumors with devastating economic consequences. The most successful bacterial biocontrol agent, nonpathogenicA. radiobacter strain K84, prevents disease by production of the BTrojan horse[ toxin agrocin 84 (Fig. 1A) (1). Because it imitates a tumor-derived substrate Eagrocinopine A (fig. S1)^, agrocin 84 isspecifically imported into A. tumefaciens strains that harbor certain types of tumor-inducing (Ti) plasmids. A toxic moiety is released from agrocin 84 (Fig. 1A) that inhibits the pathogen by anunknown mechanism (2). Agrocin 84 has a 9-(3¶-deoxy-b-D-2,3threopentafuranosyl) adenine nucleoside-like core linked to two substituents by phosphoramidate bonds (1). A 5¶-phosphoramidate bond links thenucleoside-like core to a Dthreo-2,3-dihydroxy-4-methylpentanamide, while a second phosphoramidate bond links a D-glucofuranosyloxyphosphoryl group to the adenine base and is the only known example of a6N phosphoramidate bond found in nature (3). Although this moiety is required for the selective uptake of agrocin 84 into susceptible A. tumefaciens cells, it is not required for toxicity (2). PlasmidpAgK84 in strain K84 contains the genes for agrocin 84 production and two immunity elements (4). The translation product of one of these immunity genes, agnB2, showed 940% sequence identity betweenits coding sequence and many leucyl-tRNA synthetases (LeuRSs). LeuRSs catalyze attachment of leucine to its cognate tRNAs in the first step of protein synthesis (aminoacylation). Aminoacylation assaysshowed the recombinant AgnB2 protein exhibits robust LeuRS activity (5). Importantly, the agnB2 gene is not essential for growth (6). The structure of the toxic moiety of agrocin 84 is similar to...