tokens: Single substitutions of three highly conserved phenylalanine residues ( Phe - 15 , Phe - 17 , Phe - 27 ) by alanine and substitution of one histidine ( His - 29 ) by glutamine , all located within the putative RNA - binding sites RNP - 1 and RNP - 2 , abolished the nucleic acid - binding activity of CspB .