Substrate recognition by class I lysyl-tRNA synthetases: A molecular basisfor gene displacement

Lysyl-tRNA synthetases (LysRSs) are unique amongst the aminoacyl-tRNA synth etases in being composed of unrelated class I and class II enzymes, To allo w direct comparison between the two types of LysRS, substrate recognition b y class I LysRSs was examined. Genes encoding both an archaeal and a bacter ial class I enzyme were able to rescue an Escherichia coil strain deficient in LysRS, indicating their ability to functionally substitute for a class LI LysRS in vivo, In vitro characterization showed lysine activation and re cognition to be tRNA-dependent, an attribute of several class I, but not cl ass II, aminoacyl-tRNA synthetases, Examination of tRNA recognition showed that class I LysRSs recognize the same elements in tRNA(Lys) as their class II counterparts, namely the discriminator base (N73) and the anticodon, Th is sequence-specific recognition of the same nucleotides in tRNA(Lys) by th e two unrelated types of enzyme suggests that tRNA(Lys) predates at least o ne of the LysRSs in the evolution of the translational apparatus, The only observed variation in recognition was that the G2.U71 wobble pair of spiroc hete tRNA(Lys) acts as antideterminant for class II LysRS but does not alte r class I enzyme recognition. This difference in tRNA recognition strongly favors the use of a class I-type enzyme to aminoacylate particular tRNA(Lys ) species and provides a molecular basis for the observed displacement of c lass II by class I LysRSs in certain bacteria.