5-S RNA-BINDING PROTEIN FROM YEAST (SACCHAROMYCES-CEREVISIAE) RIBOSOMES - EVOLUTION OF THE EUKARYOTIC 5-S RNA-BINDING PROTEIN

The ribonucleoprotein complex between 5‐S RNA and its binding protein (5‐S RNA · protein complex) of yeast ribosomes was released from 60‐S subunits with 25 mM EDTA and the protein component was purified by chromatography on DEAE‐cellulose. This protein, designated YL3 (Mr= 36000 on dodecylsulfate gels), was relatively insoluble in neutral solutions (pH 4–9) and migrated as one of four acidic 60‐S subunit proteins when analyzed by the Kaltschmidt and Wittman two‐dimensional gel system. Amino acid analyses indicated lower amounts of lysine and arginine than most ribosomal proteins. Sequence homology was observed in the N terminus of YL3, and two prokaryotic 5‐S RNA binding proteins, EL 18 from Escherichia coli and HL13 from Halobacterium cutirubrum:Ala1‐Phe2‐Gln3‐Lys4‐Asp5‐Ala6‐Lys7‐Ser8‐Ser9‐Ala10‐Tyr11‐Ser12‐Ser13‐Arg14‐Phe15‐Gln16‐Tyr17‐Pro18‐Phe19‐Arg20‐Arg21‐Arg22‐Arg23‐Glu24‐Gly25‐Lys26‐Thr27‐Asp28‐Tyr29‐Tyr3s;of particular interest was homology in the cluster of basic residues (18–23). Since the protein contained one methionine residue it could be split into two fragments, CN1 (Mr= 24700) and CN2 (Mr= 11300) by CNBr treatment; the larger fragment originated from the N terminus. The N‐terminal amino acid sequence of CN2 shared a limited sequence homology with an internal portion of a second 5‐S RNA binding protein from E. coli, EL5, and, based also on the molecular weights of the proteins and studies on the protein binding sites in 5‐S RNAs, a model for the evolution of the eukaryotic 5‐S RNA binding protein is suggested in which a fusion of the prokaryotic sequences may have occurred. Unlike the native 5‐S RNA · protein complex, a variety of RNAs interacted with the smaller CN2 fragment to form homogeneous ribonucleoprotein complexes; the results suggest that the CN1 fragment may confer specificity on the natural 5‐S RNA‐protein interaction. Copyright © 1979, Wiley Blackwell. All rights reserved