GROUP I introns are highly structured RNAs which catalyse their own splicing by guanosine-initiated transesterification reactions(1,2). Their catalytic core is generally stabilized by RNA-RNA interactions within the core and with peripheral RNA structures(3,4). Additionally, some group I introns require proteins for efficient splicing in vivo(5). The Neurospora CYT-18 protein, the mitochondrial tyrosyl-transfer RNA synthetase (mt TyrRS), promotes splicing of the Neurospora mitochondrial large ribosomal RNA (LSU) and other group I introns by stabilizing the catalytically active structure of the intron core(6-8). We report here that CYT-18 functions similarly to a peripheral RNA structure, P5abc, that stabilizes the catalytic core of the Tetrahymena LSU intron. The CYT-18 protein and P5abc RNA bind to overlapping sites in the intron core, inducing similar conformational changes correlated with splicing activity. Our results show that a protein can play the role of an RNA structure in a catalytic RNA, a substitution postulated for the evolution of nuclear pre-messenger RNA introns from self-splicing introns(9,10).