Identification and analysis of candidate fungal tRNA 3'-end processing endonucleases tRNase Zs, homologs of the putative prostate cancer susceptibility protein ELAC2.

BACKGROUND

tRNase Z is the endonuclease that is responsible for the 3'-end processing of tRNA precursors, a process essential for tRNA 3'-CCA addition and subsequent tRNA aminoacylation. Based on their sizes, tRNase Zs can be divided into the long (tRNase ZL) and short (tRNase ZS) forms. tRNase ZL is thought to have arisen from a tandem gene duplication of tRNase ZS with further sequence divergence. The species distribution of tRNase Z is complex. Fungi represent an evolutionarily diverse group of eukaryotes. The recent proliferation of fungal genome sequences provides an opportunity to explore the structural and functional diversity of eukaryotic tRNase Zs.

RESULTS

We report a survey and analysis of candidate tRNase Zs in 84 completed fungal genomes, spanning a broad diversity of fungi. We find that tRNase ZL is present in all fungi we have examined, whereas tRNase ZS exists only in the fungal phyla Basidiomycota, Chytridiomycota and Zygomycota. Furthermore, we find that unlike the Pezizomycotina and Saccharomycotina, which contain a single tRNase ZL, Schizosaccharomyces fission yeasts (Taphrinomycotina) contain two tRNase ZLs encoded by two different tRNase ZL genes. These two tRNase ZLs are most likely localized to the nucleus and mitochondria, respectively, suggesting partitioning of tRNase Z function between two different tRNase ZLs in fission yeasts. The fungal tRNase Z phylogeny suggests that tRNase ZSs are ancestral to tRNase ZLs. Additionally, the evolutionary relationship of fungal tRNase ZLs is generally consistent with known phylogenetic relationships among the fungal species and supports tRNase ZL gene duplication in certain fungal taxa, including Schizosaccharomyces fission yeasts. Analysis of tRNase Z protein sequences reveals putative atypical substrate binding domains in most fungal tRNase ZSs and in a subset of fungal tRNase ZLs. Finally, we demonstrate the presence of pseudo-substrate recognition and catalytic motifs at the N-terminal halves of tRNase ZLs.

CONCLUSIONS

This study describes the first comprehensive identification and sequence analysis of candidate fungal tRNase Zs. Our results support the proposal that tRNase ZL has evolved as a result of duplication and diversification of the tRNase ZS gene.