A TYROSYL-TRANSFER-RNA SYNTHETASE CAN FUNCTION SIMILARLY TO AN RNA STRUCTURE IN THE TETRAHYMENA RIBOZYME

GROUP I introns are highly structured RNAs which catalyse their own sp licing by guanosine-initiated transesterification reactions(1,2). Thei r catalytic core is generally stabilized by RNA-RNA interactions withi n the core and with peripheral RNA structures(3,4). Additionally, some group I introns require proteins for efficient splicing in vivo(5). T he Neurospora CYT-18 protein, the mitochondrial tyrosyl-transfer RNA s ynthetase (mt TyrRS), promotes splicing of the Neurospora mitochondria l large ribosomal RNA (LSU) and other group I introns by stabilizing t he catalytically active structure of the intron core(6-8). We report h ere that CYT-18 functions similarly to a peripheral RNA structure, P5a bc, that stabilizes the catalytic core of the Tetrahymena LSU intron. The CYT-18 protein and P5abc RNA bind to overlapping sites in the intr on core, inducing similar conformational changes correlated with splic ing activity. Our results show that a protein can play the role of an RNA structure in a catalytic RNA, a substitution postulated for the ev olution of nuclear pre-messenger RNA introns from self-splicing intron s(9,10).