Recent Studies suggest that some RNA-binding proteins facilitate the folding of non-cognate RNAs. Here, we report that bacteriophage MS2 coat protein (MS2 CP) bound and promoted the catalytic activity of Candida group I ribozyme. Cloning of the MS2-bound RNA segments showed that this protein primarily interacts with the P5ab-P5 Structure. Ultraviolet cross-linking and the T1 footprinting assay further showed that MS2 binding stabilized tertiary interactions, including the conserved L9-P5 interaction, and led to a more compact Core Structure, This mechanism is similar to that of the yeast mitochondrial tyrosyl-tRNA synthetase on other group I introns, suggesting that different RNA-binding Proteins May use common mechanisms to support RNA Structures. (C) 2008 Elsevier Inc. All rights reserved.