High-level biosynthetic substitution of methionine in proteins by its analogs 2-aminohexanoic acid, selenomethionine, telluromethionine and ethionine in Escherichia coli.

We have utilized a T7 polymerase/promoter system for the high-level incorporation of methionine analogs with suitable labels for structural research (X-ray and NMR studies) on recombinant annexin V produced in Escherichia coli. Here, we describe, to our knowledge, the first biosynthetic high-level substitution of methionine by 2-aminohexanoic acid (norleucine), ethionine and telluromethionine in a protein. The replacement has been confirmed by electrospray mass spectroscopy, amino acid analysis and X-ray structural analysis. Conditions for expression were optimized concerning the frequency of appearance of revertants, high-level replacement and maximal protein yield. For the incorporation of norleucine and ethionine, E. coli B834 (DE3)(hsd metB), which is auxotrophic for methionine, was grown under methionine-limited conditions with an excess of the analog in the culture medium, and the expression of protein under the control of the T7 promoter was induced after the methionine supply had been exhausted. The factor limiting the high-level incorporation of telluromethionine into protein is its sensitivity towards oxidation. To overcome this problem, bacteria were grown with a limited amount of methionine, harvested after its exhaustion and resuspended in fresh media without methionine; telluromethionine was added and protein synthesis induced. Under these conditions, significant amounts of protein can be expressed before telluromethionine has been completely degraded (within hours). Biosynthetic incorporation of heavy atoms such as tellurium into recombinant proteins can accelerate the process of obtaining heavy-atom derivatives suitable for X-ray structural analysis, supplementing the traditional trial-and-error preparation of heavy-atom derivatives for the method of multiple isomorphous replacement. Furthermore, the successful high-level incorporation of amino acid analogs can provide single-atom mutations for the detailed study of the structure and function of proteins.