Genomic and evolutionary aspects of chloroplast tRNA in monocot plants

Chloroplasts are one of the most indispensable organelles that make life forms on the earth possible by their capacity to photosynthesize. These organelles possess a circular genome with a number of coding genes responsible for self-regulation. tRNAs are an important evolutionary-conserved gene family that are responsible for protein translation. However, within the chloroplast genome, tRNA machinery are poorly understood.

In the present study, the chloroplast genome of six monocot plants, Oryza nivara (NC_005973), Oryza sativa (NC_001320), Sachharum officinarum (NC_006084), Sorghum bicolor (NC_008602), Triticum aestivum (NC_002762), and Zea mays (NC_001666) were downloaded and analyzed to identify tRNA sequences. Further analysis of the tRNA sequences in the chloroplast genomes of the monocot plants resulted in the identification of several novel features. The length of tRNAs in the chloroplast genome of the monocot plants ranged from 59 to 155 nucleotides. Pair-wise sequence alignment revealed the presence of a conserved A-C-x-U-A-x-U-A-x-U-x₅-U-A-A nucleotide consensus sequence. In addition, the tRNAs in chloroplast genomes of the monocot plants also contain 21-28 anti-codons against 61 sense codons in the genome. They also contain a group I intron and a C-A-U anti-codon for tRNA^Ile, which is a common anti-codon of tRNA^Met. Evolutionary analysis indicates that tRNAs in the chloroplast genome have evolved from multiple common ancestors, and tRNA^Met appears to be the ancestral tRNA that underwent duplication and diversification to give rise to other tRNAs.

The results obtained from the study of chloroplast tRNA will greatly help to increase our understanding of tRNA biology at a new level. Functional studies of the reported novel aspects of the chloroplast tRNA of the monocot plants will greatly help to decipher their roles in diverse cellular processes.