The CUG codon is decoded in vivo as serine and not leucine in Candida albicans

摘要

Previous studies have shown that the yeast Candida albicans encodes a unique seryl-tRNA_CAG that should decode the leucine codon CUG as serine. However, In vitro translation of several different CUG-contalnlng mRNAs in the presence of this unusual seryl-tRNA_CAG result in an apparent increase in the molecular weight of the encoded polypeptldes as Judged by SDS-PAGE even though the molecular weight of serine is lower than that of leucine. A possible explanation for this altered electrophoretic mobility is that the CUG codon is decoded as modified serine In vitro. To elucidate the nature of CUG decoding In vivo, a reporter system based on the C.alblcans gene (RBP1) encoding ra-pamycin-binding protein (RBP), coupled to the promoter of the C.alblcans TEF3 gene, was utilized. Sequencing and mass-spectrometry analysis of the recomblnant RBP expressed in C.alblcans demonstrated that the CUG codon was decoded exclusively as serine while the related CUU codon was translated as leucine. A database search revealed that 32 out of the 65 C.alblcans gene sequences available have CUG codons in their open reading frames. The CUG-containing genes do not belong to any particular gene family. Thus the amlno acid specified by the CUG codon has been reassigned within the mRNAs of C.albicans. We argue here that this unique genetic code change In cellular mRNAs cannot be explained by the ‘Codon Reassignment Theory'.