title: Transfer RNA Is the Source of Extracellular Isopentenyladenine in a Ti-Plasmidless Strain of Agrobacterium tumefaciens.
abstract: Even in the absence of the classical Ti plasmid-encoded cytokinin biosynthetic genes ipt and tzs, Agrobacterium tumefaciens strains still release significant amounts of the cytokinin isopentenyladenine (iP) into the culture medium (R.W. Kaiss-Chapman and R.O. Morris [1977] Biochem Biophys Res Commun 76: 453-459). A potential source of the iP is isopentenylated transfer RNA (tRNA), which, in turn, is synthesized by the activity of tRNA:isopentenyltransferase encoded by the bacterial miaA gene. To determine whether secreted iP had its origin in isopentenylated tRNA, a miaA- deletion/insertion mutant was prepared and reconstructed in Agrobacterium tumefaciens in vivo. The mutant no longer possessed tRNA:isopentenylation activity and no longer released iP into the extracellular medium. Transfer RNA therefore makes a small but significant contribution to the total amount of cytokinin normally secreted by Agrobacterium strains. tRNA-mediated synthesis may also account for cytokinin production by other plant-associated bacteria, such as Rhizobia, that have been reported to secrete similarly low levels of nonhydroxylated cytokinins.
authors.affiliation: Department of Biochemistry, University of Missouri, Columbia, Missouri 65211-0001 (J.G., R.M., R.O.M.).
authors.forename: J.
authors.initials: J
authors.lastname: Gray
authors.forename: S. B.
authors.initials: SB
authors.lastname: Gelvin
authors.forename: R.
authors.initials: R
authors.lastname: Meilan
authors.forename: R. O.
authors.initials: RO
authors.lastname: Morris