Activation factor of RTCB, the catalytic subunit of the tRNA- splicing ligase complex (tRNA-LC), which is involved in the enzyme- dependent maturation of intron-containing pre-tRNAs. Functions downstream of the tRNA-splicing endonuclease that removes introns, ligating the two generated halves via phosphodiester bond formation. Catalyzes the GTP-dependent activation of RTCB, both proteins forming a composite active site coordinating GTP and metal ions to enable the guanylylation of RTCB a prerequisite to undergo multiple catalytic cycles (PubMed:24870230, PubMed:38493148). Also required for the activation of RTCB in non-canonical, spliceosome-independent, cytoplasmic splicing of XBP1 mRNAs during the unfolded protein response (UPR). Upon endoplasmic reticulum (ER) stress, the endoribonuclease IRE1/ERN1 excises a short intron, generating free exon ends that are aligned by RNA-intrinsic, zipper-like stem-loop structures. These exon ends are then recognized and ligated by RTCB. This splicing event yields the active XBP1s transcription factor, which induces genes required to resolve protein folding defects in the endoplasmic reticulum (PubMed:25378478, PubMed:26483401, PubMed:38493148). {ECO:0000269|PubMed:24870230, ECO:0000269|PubMed:25378478, ECO:0000269|PubMed:26483401, ECO:0000269|PubMed:38493148}. This is the function of TSLIG1 (tRNA splicing ligase complex subunit 1, Ensembl gene identifier ENSG00000176261).