Nucleotide sequence of a spinach chloroplast threonine tRNA.

The nucleotide sequence of a spinach chloroplast threonine tRNA has been determined. This chloroplast threonine tRNA has been determined. This chloroplast threonine tRNA has 75 nucleotides, which is the same chain length as the recently determined threonine tRNA from yeast mitochondria. This contrasts with the 6 non-organelle threonine tRNAs sequenced to date, which are 76 nucleotides in length. However, other than this similarity in size, the chloropast tRNAThr has little similarlity to the yeast mitochondrial threonine tRNA, and shows essentially equal homology to both prokaryotic and eukaryotic feature of sequence homology to other threonine tRNAs, there is a 19-nucleotide segment encompassing the entire T psi C stem and loop region, that, except for post-transcriptional modifications, is identical in this spinach chloroplast tRNAThr and in yeast cytoplasmic tRNAThr1A. A most unusual feature of this spinach chloroplast tRNAThr is that it has an A residue at the 5'-end of the anticodon loop, a site that is occupied by a pyrimidine in all other tRNAs sequenced to date. Other than this feature, the tRNAThr contains all of the invariant and semi-invariant residues normally found in tRNAs. This tRNA hybridizes to spinach chloroplast DNA, but does not hybridize to the same region of the spinach chloroplast genome that contains the genes for spinach chloroplast tRNA1Thr or tRNA2Thr. This tRNA therefore appears to be a third isoaccepting species of threonine tRNA encoded by spinach chloroplast DNA.