Catalytic subunit of the tRNA-splicing ligase complex that acts by directly joining spliced tRNA halves to mature-sized tRNAs by incorporating the precursor-derived splice junction phosphate into the mature tRNA as a canonical 3',5'-phosphodiester. May act as an RNA ligase with broad substrate specificity, and may function toward other RNAs (By similarity). Essential during post-implantation development of embryos. a 3'-end 3'-phospho-ribonucleotide-RNA + a 5'-end dephospho-ribonucleoside-RNA + GTP = a ribonucleotidyl-ribonucleotide-RNA + diphosphate + GMP a 3'-end 2',3'-cyclophospho-ribonucleotide-RNA + a 5'-end dephospho-ribonucleoside-RNA + GTP + H2O = a ribonucleotidyl-ribonucleotide-RNA + diphosphate + GMP + H(+) Binds 2 manganese ions per subunit. Catalytic component of the tRNA-splicing ligase complex. Enters into the nucleus in case of active transcription while it accumulates in cytosol when transcription level is low. Ubiquitously expressed. Highly expressed in the epididymis with predominant enrichment in the initial segment. During sexual maturation, it is expressed in the caput epididymides. Down-regulated in the epididymis upon castration. Ligation probably proceeds through 3 nucleotidyl transfer steps, with 2',3'-cyclic phosphate termini being hydrolyzed to 3'-P termini in a step that precedes 3'-P activation with GMP. In the first nucleotidyl transfer step, RTCB reacts with GTP to form a covalent RTCB-histidine-GMP intermediate with release of PPi; in the second step, the GMP moiety is transferred to the RNA 3'-P; in the third step, the 5'-OH from the opposite RNA strand attacks the activated 3'-P to form a 3',5'-phosphodiester bond and release GMP. Belongs to the RtcB family.