Catalytic subunit of the tRNA-splicing ligase complex that acts by directly joining spliced tRNA halves to mature-sized tRNAs (PubMed:25366321). Required for the ligation of mRNAs and specifically, regulates xbp-1 mRNA splicing during the endoplasmic reticulum stress-induced unfolded protein response (PubMed:25366321, PubMed:25429148). Has a neuroprotective role in the age-dependent degeneration of dopamine neurons, which is mediated by xbp-1 (PubMed:25429148). 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. Mutants are sterile and few are viable (PubMed:25366321, PubMed:25429148). Mutants have a slower growth rate, reduced lifespan and have defective vulval development displaying a severe protruding vulva phenotype (PubMed:25366321, PubMed:25429148). Mutants have disrupted RNA ligase activity which results in the presence of unligated tRNAs and defective tRNA processing (PubMed:25366321). RNAi-mediated knockdown results in reduced sensitivity to tunicamycin-induced endoplasmic reticulum (ER) stress and significantly reduced levels of spliced xbp-1 mRNA under tunicamycin-induced ER stress and non-stressed conditions (PubMed:25429148). RNAi-mediated knockdown in dopamine neurons enhances degenerative effects induced by alpha-synuclein and the neurotoxin, 6-OHDA (PubMed:25429148). 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.