Cloning and Nucleotide Sequence of a Tobacco Chloroplast Translational Elongation Factor, EF-Tu

Translational elongation factor is an essential component for protein synthesis that functions by binding aminoacylated tRNAs to the ribosome-mRNA complex. This function is performed by EF-Tu in prokaryotes and eukaryotic organelles, and by EF-la in eukaryotes (for review see Miller and Weissbach, 1977). In lower photosynthetic eukaryotes, including Chlamydomonas (Baldauf and Palmer, 1990) and Euglena grad i s (Montadon and Stutz, 1983), EF-Tu is encoded by the chloroplast genome. In higher plants, an evolutionary transfer of this chloroplast gene to the nucleus has occurred (Baldauf and Palmer, 1990). In higher plants, homology between EF-Tu and the functionally similar cytoplasmic elongation factor, EF-la, is significantly less than homology between EF-Tu and the bacterial elongation factor, tufA (Baldauf and Palmer, 1990; V.M. Ursin, unpublished observation). From a tobacco genomic library, we have cloned a gene encoding tobacco EF-Tu (NtEFTu) utilizing sequence information from the Arabidopsis homolog ATHTufA (Baldauf and Palmer, 1990) (Table I). The genomic fragment cloned contains an open reading frame of 1444 bp encoding a 478amino acid protein (Fig. 1). The deduced polypeptide contains a 69-amino acid serine-rich chloroplast transit peptide, and the predicted molecular mass of the mature protein is approximately 44.6 kD. Sequence identity between the mature Arabidopsis and tobacco genes is high along their entire coding regions, with complete conservation observed within a 30-amino acid region flanking cysteine-151 (at position 81 in E. g r a d i s ) that is required for the interaction of the protein with aminoacyl tRNA (Miller and Weissbach, 1977). Neither the NtEFTu nor ATHTufA genes contains introns, which is in contrast with the highly homologous E. g rad i s EF-Tu gene, which contains two small introns within its coding region (Montadon and Stutz, 1983). Table 1. Characteristics of the EF-TU gene from tobacco