6 NEW CANDIDATE MEMBERS OF THE ALPHA BETA TWISTED OPEN-SHEET FAMILY DETECTED BY SEQUENCE SIMILARITY TO FLAVODOXIN/

Strong sequence similarity has been reported among WrbA (the Trp repre ssor-binding protein of Escherichia coli); Ycp4, a protein of unknown function from the budding yeast Saccharomyces cerevisiae; P25, the pap 1-dependent protein of the fission yeast Schizosaccharomyces pombe; a nd the translation product of a partial cDNA sequence from rice seedli ng root (Oryza sativa, locus Ricr02421a; here referred to as RicR). Fu rther homology search with the profile method indicates that all the a bove sequences are related to the flavodoxin family and, in turn, allo ws detection of the recently proposed flavodoxin-like proteins from E. coli, MioC and the hypothetical protein YihB. We discuss sequence con servation with reference to the known 3-dimensional structures of flav odoxins. Conserved sequence and hydrophobicity patterns, as well as re sidue-pair interaction potentials, strongly support the hypothesis tha t these proteins share the alpha/beta twisted open-sheet fold typical of flavodoxins, with an additional alpha/beta unit in the WrbA family. On the basis of the proposed structural homology, we discuss the deta ils of the putative FMN-binding sites. Our analysis also suggests that the helix-turn-helix motif we identified previously in the C-terminal region of the WrbA family is unlikely to reflect a DNA-binding functi on of this new protein family.