STUDY OF TYROSINE-CONTAINING MUTANTS OF RIBOSOMAL-PROTEIN L7 L12 FROMESCHERICHIA-COLI/

Three mutant forms of the ribosomal protein L7/L12 with replacements o f Ser1, Met14 and Met26 to Tyr were studied by the methods of fluoresc ence spectroscopy, circular dichroism and microcalorimetry. The amino- acid residue Tyr14 in the protein L7/L12 Tyr14 is located in a region with a more organized structure than Tyr26 in protein L7/L12 Tyr26. Th e replacements Ser1 --> Tyr1 and Met14 --> Tyr14 do not affect the sec ondary structure of protein L7/L12. The replacement Met26 --> Tyr26 st abilizes the secondary structure of protein L7/L12. A pH-induced tempe rature transition was observed in the pH range 5.0-7.3 in protein L7/L 12 Tyr14 by tyrosine fluorescence. Analogous transitions were observed for protein L7/L12 Tyr26 by Tyr fluorescence and for the wild type pr otein L7/L12 by Phe fluorescence. Three pH-dependent states of protein L7/L12 and its mutant forms L7/L12 Tyr1 and L7/L12 Tyr14 were found o n the microcalorimetric melting curves, The characteristics of protein L7/L12 Tyr14 are very close to the wild type protein L7/L12 and it is a suitable object for studying the structure of the N-terminal part o f molecule by two-dimentional H-1-NMR.