The antibiotic kirromycin (Kr) inhibits bacterial protein synthesis by binding to elongation factor Tu (EF-Tu). and producers of antibiotics of the Kr class, are known to possess an EF-Tu resistant to Kr. Both micro-organisms appear to possess a single gene and we have characterized the one from which belongs to the family. To assess the molecular determinants of Kr resistance, the gene was expressed in as a translational fusion to which enabled the recovery by affinity chromatography of the recombinant protein uncontaminated by the host factor. The recombinant EF-Tu was able to catalyse polyU-directed polyPhe synthesis in two heterologous cell-free systems, even as an uncleaved fusion. When tested for antibiotic sensitivity it behaved like the natural protein, showing equivalent resistance to Kr but sensitivity to the antibiotic GE2270, indicating that all determinants for Kr resistance are intrinsic to the EF-Tu sequence. Multiple sequence analysis of EF-Tu proteins, together with knowledge of mutations conferring Kr resistance, allowed the identification of key residues as likely candidates for the natural Kr resistance of the EF-Tu. One of these, Thr was mutated to the consensus Ala and the resulting mutant protein was sensitive to Kr. Interestingly, it retained some activity (30% of the control) even at. high Kr concentrations.


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