On the evolution and expression of Chlamydomonas reinhardtii nucleus-encoded transfer RNA genes

In Chlamydomonas reinhardtii, 259 tRNA genes were identified and classified into 49 tRNA isoaccepting families. By constructing phylogenetic trees, we determined the evolutionary history for each tRNA gene family. The majority of the IRNA sequences are more closely related to their plant counterparts than to animals ones. Northern experiments also permitted LIS to show that at least one member of each IRNA isoacceptor family is transcribed and correctly processed in vivo. A short stretch of T residues known to be a signal for termination of polymerase III transcription was found downstream of most IRNA genes. It allowed us to propose that the vast majority of the IRNA genes are expressed and to confirm that numerous IRNA genes separated by short spacers are indeed cotranscribed. Interestingly, in silico analyses and hybridization experiments show that the cellular IRNA abundance is correlated with the number of tRTNA genes and is adjusted to the codon usage to optimize translation efficiency. Finally, we studied the origin of SINEs, short interspersed elements related to tRNAs, whose presence in Chlamydomonas is exceptional. Phylogenetic analysis strongly suggests that tRNA(Asp)-related SINEs originate front a prokaryotic-type IRNA either horizontally transferred from a bacterium or originally present in mitochondria or chloroplasts.