Component of the EKC/KEOPS complex that is required for the formation of a threonylcarbamoyl group on adenosine at position 37 (t(6)A37) in tRNAs that read codons beginning with adenine. The complex is probably involved in the transfer of the threonylcarbamoyl moiety of threonylcarbamoyl-AMP (TC-AMP) to the N6 group of A37. Cgi121 acts as an allosteric effector that regulates the t(6)A activity of the complex. The EKC/KEOPS complex also promotes both telomere uncapping and telomere elongation. The complex is required for efficient recruitment of transcriptional coactivators. Cgi121 is not required for tRNA modification (By similarity). Component of the EKC/KEOPS complex composed of at least bud32, cgi121, gon7, kae1 and pcc1; the whole complex dimerizes. Belongs to the CGI121/TPRKB family.