To obtain an RNA-modifying protein capable of changing the structure of an mRNA in a direction for improving a protein synthesis efficiency, a protein synthesis-promoting agent easily improving the protein synthesis efficiency, a gene therapeutic agent easily enhancing a translational efficiency of a transgene, and a cancer-treating agent capable of performing the treatment of cancer easily and effectively.

The RNA-modifying protein has a translational structure specific RNAase activity excising a sequence in a non-translational zone (5'UTR, 3'UTR) in the mRNA and an RNA-unwinding activity for unwinding the mRNA having a higher structure to stabilize it at a primary structure. The protein synthesis- promoting agent contains the RNA-modifying protein or a gene encoding the protein. The gene therapeutic agent contains the transgene and the protein synthesis-promoting agent. The cancer-treating agent contains the RNA- modifying protein, an antibody against a partial peptide of the protein or an anti-sense nucleotide against the protein or a gene encoding a part of it.