CDC60 was first identified as a temperature sensitive mutant that arrested at START upon shift to the restrictive temperature (1). It encodes the cytoplasmic leucyl-tRNA synthase (2). The cell cycle arrest of the mutant is probably due to the block in protein synthesis that results from a lack of charged leucyl-tRNA (2). CDC60 is 50% identical to the Neurospora crassa cytosolic leucyl-tRNA synthase, and has regions of 19-21% identity to E. coli leucyl-, isoleucyl-, and methionyl-tRNA synthase (2). The yeast Cdc60p can aminoacylate E.coli tRNA-Leu (3).

About aminoacyl-tRNA synthetases...

In a process critical for accurate translation of the genetic code, aminoacyl-tRNA synthetases (aka aminoacyl-tRNA ligases) attach amino acids specifically to cognate tRNAs, thereby "charging" the tRNAs. The catalysis is accomplished via a two-step mechanism. First, the synthetase activates the amino acid in an ATP-dependent reaction, producing aminoacyl-adenylate and releasing inorganic pyrophosphate (PPi). Second, the enzyme binds the correct tRNA and transfers the activated amino acid to either the 2' or 3' terminal hydroxyl group of the tRNA, forming the aminoacyl-tRNA and AMP (4, 5 and references therein).

Aminoacyl-tRNA synthetases possess precise substrate specificity and, despite their similarity in function, vary in size, primary sequence and subunit composition. Individual members of the aminoacyl-tRNA synthetase family can be categorized in one of two classes, depending on amino acid specificity. Class I enzymes (those specific for Glu, Gln, Arg, Cys, Met, Val, Ile, Leu, Tyr and Trp) typically contain two highly conserved sequence motifs, are monomeric or dimeric, and aminoacylate at the 2' terminal hydroxyl of the appropriate tRNA. Class II enzymes (those specific for Gly, Ala, Pro, Ser, Thr, His, Asp, Asn, Lys and Phe) typically contain three highly conserved sequence motifs, are dimeric or tetrameric, and aminoacylate at the 3' terminal hydroxyl of the appropriate tRNA (4, 5, 6 and references therein).

Last updated: 1999-09-01