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. 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. Protein archease stimulates the activity of the tRNA ligase complex with high efficiency in the presence of GTP. 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. 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.