TRANSFER-RNA IS THE SOURCE OF EXTRACELLULAR ISOPENTENYLADENINE IN A TI-PLASMIDLESS STRAIN OF AGROBACTERIUM-TUMEFACIENS

Even in the absence of the classical Ti plasmid-encoded cytokinin bios ynthetic genes ipt and tzs, Agrobacterium tumefaciens strains still re lease significant amounts of the cytokinin isopentenyladenine (iP) int o the culture medium (R.W. Kaiss-Chapman and R.O. Morris [1977] Bioche m Biophys Res Commun 76: 453-459). A potential source of the iP is iso pentenylated transfer RNA (tRNA), which, in turn, is synthesized by th e activity of tRNA:isopentenyltransferase encoded by the bacterial mia A gene. To determine whether secreted iP had its origin in isopentenyl ated tRNA, a miaA(-) deletion/insertion mutant was prepared and recons tructed in Agrobacterium tumefaciens in vivo. The mutant no longer pos sessed tRNA:isopentenylation activity and no longer released iP into t he extracellular medium. Transfer RNA therefore makes a small but sign ificant contribution to the total amount of cytokinin normally secrete d by Agrobacterium strains. tRNA-mediated synthesis may also account f or cytokinin production by other plant-associated bacteria, such as Rh izobia, that have been reported to secrete similarly low levels of non hydroxylated cytokinins.