Unique pathway of expression of an opal suppressor phosphoserine tRNA.

An opal suppressor phosphoserine tRNA gene is present in single copy in the genomes of higher vertebrates. We have shown that the product of this gene functions as a suppressor in an in vitro assay, and we have proposed that it may donate a modified amino acid directly to protein in response to specific UGA codons. In this report, we show through in vitro and in vivo studies that the human and Xenopus opal suppressor phosphoserine tRNAs are synthesized by a pathway that is, to the best of our knowledge, unlike that of any known eukaryotic tRNA. The primary transcript of this gene does not contain a 5'-leader sequence; and, therefore, transcription of this suppressor is initiated at the first nucleotide within the coding sequence. The 5'-terminal triphosphate, present on the primary transcript, remains intact through 3'-terminal maturation and through subsequent transport of the tRNA to the cytoplasm. The unique biosynthetic pathway of this opal suppressor may underlie its distinctive role in eukaryotic cells.