Differential use of termination codons in ciliated protozoa

Proc Natl Acad Sci U S A. 1989 May;86(9):3252-6. doi: 10.1073/pnas.86.9.3252.


Sequence analysis of genes in four species of ciliated protozoa and analysis of tRNAs in Tetrahymena has demonstrated that TAG and TAA encode glutamine or glutamic acid in these organisms and TGA is the only stop codon. Thus, it has generally been assumed that all ciliates use a nonuniversal genetic code in which TGA acts as the sole termination codon. We have sequenced the linear DNA molecules that carry an actin gene and a beta-tubulin gene from the ciliate Euplotes crassus. These genes are shown to use TAA as a termination codon based on homology to known actin and beta-tubulin gene sequences. In addition, we have sequenced a portion of the 3' terminus of the E. crassus H4 histone gene and show that it also uses TAA as a termination codon. These data indicate that the timing of genetic code changes in the ciliates must be reconsidered.

Publication types

  • Comparative Study
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Actins / genetics
  • Amino Acid Sequence
  • Animals
  • Base Sequence
  • Biological Evolution
  • Ciliophora / genetics*
  • Codon*
  • DNA / genetics*
  • Genetic Code
  • Glutamates / genetics
  • Glutamic Acid
  • Glutamine / genetics
  • Histones / genetics
  • Molecular Sequence Data
  • Nucleic Acid Hybridization
  • RNA / genetics
  • RNA, Messenger*
  • Species Specificity
  • Tetrahymena / genetics
  • Tubulin / genetics


  • Actins
  • Codon
  • Glutamates
  • Histones
  • RNA, Messenger
  • Tubulin
  • Glutamine
  • Glutamic Acid
  • RNA
  • DNA

Associated data

  • GENBANK/J04533
  • GENBANK/J04534