| Archaeal and Eukaryal introns are removed by the step-wise action of two enzymes---an endonuclease and a ligase---towards the end of tRNA maturation. The endonuclease cleavage pathway is conserved in both archaea and eukarya. There are three types of archaeal endonucleases: homodimeric, homotetrameric, and the newly discovered heterotetrameric. Most tRNA introns in archaea are located in the anticodon loop, at position 37/38, but introns also have been found at 14 other locations in pre-tRNA, mostly in crenarchaeota. A perfect BHB motif (hBHBh') recognizable to the archaeal endonuclease is not always formed at the intron-exon junction. Instead, a relaxed or imperfect motif (hBH or HBh') may be formed, which includes the central helix (H) and one of the flanking helices (h or h'). This study reports on the processing of imperfect BHB motif containing tRNAs by two types of archaeal endonucleases. |
