Fluorescence properties of a tryptophan residue in an aromatic core of the protein subunit of ribonuclease P from Escherichia coli

Escherichia coli ribonuclease P (RNase P), a ribonucleoprotein complex which primarily functions in tRNA biosynthesis, is composed of a catalytic RNA subunit, M1 RNA, and a protein cofactor, C5 protein. The fluorescence emission spectrum of the single tryptophan residue-containing C5 protein exhibits maxima at 318 nm and 332 nm. Based on a comparison of the emission spectra of wild-type C5 protein and some of its mutant derivatives, we have determined that the 318 nm maximum could be the result of a complex formed in the excited state as a result of hydrophobic interactions between Trp109, Phe18 and Phe73. The analogous tryptophan fluorescence emission spectra of wild-type C5 protein and the barstar mutant W38F/W44F, taken together with the detailed structural information available for barstar, provide a possible explanation for the unusual emission spectrum of C5 protein.