Homology between chloroplast and prokaryotic initiator tRNA. Nucleotide sequence of spinach chloroplast methionine initiator tRNA.

The nucleotide sequence of a chloroplast methionine initiator tRNA from spinach has been determined. Although from a eukaryotic organism, this tRNA strongly resembles prokaryotic initiator tRNAs. Spinach chloroplast tRNAMetf has a much higher sequence homology with prokaryotic initiator tRNAs (81 to 84%) than with eukaryotic initiator tRNAs (64 to 69%). In addition, it possesses the two unique features of prokaryotic initiator tRNAs, lacking a base pair between the 5'-terminal residue and the fifth nucleotide from the 3'-end and containing a T-psi-C-A sequence in loop IV. Also, like prokaryotic initiator tRNAs, the chloroplast tRNAMetf is 77 nucleotides long and has few modified nucleosides (2'-O-methylguanosine, dihydrouridine, 7-methylguanosine, ribothymidine, and pseudouridine). This chloroplast initiator tRNA is strikingly different in sequence homology (55 to 62%), number of residues, and structure from mitochondrial initiator tRNAs. Restriction enzyme mapping techniques have shown that the chloroplast tRNAMEtf hybridizes to spinach chloroplast DNA. A set of characteristic chloroplast tRNA features seems to be emerging from a comparison of this tRNAMetf and several other chloroplast tRNAs which have been completely or partially sequenced. All have a 2'-O-methylated G-G sequence in the dihydrouridine loop, and the sequence T-psi-C-A, as opposed to T-psi-C-G, is predominantly found in loop IV. This is the reverse of the situation encountered in the overall non-chloroplast tRNA population.