Resistance to fusidic acid in is due to secretion of an extracellular enzyme (FusH) that converts the steroid antibiotic into an inactive derivative. NH-terminal and several internal amino acid sequences were prepared from the purified enzyme. Using one of the deduced oligonucleotides to probe a subgenomic DNA library, the gene was cloned and sequenced. Sequence analysis located an ORF which, owing to the presence of two putative start codons, indicates a predicted protein with 520 or 509 amino acids. A signal peptide was identified by aligning the deduced amino acids with the N-terminal sequence determined for the mature enzyme. The C-terminal part of the deduced FusH contains a region of three tandemly repeated stretches of 50 amino acids, which is preceded and followed by amino acids showing high homology with the repeats. FusH was found to share a GDS motif with some deduced esterases. transformants carrying on a multicopy vector synthesized high levels of FusH. Purified FusH cleaved equally well an acetyl, a thioacetyl or a formyl group from the 16β-position of fusidic acid and its derivatives. However, a propionyl group at the 16β-position was attacked with difficulty and a 16β-acetyl group was not hydrolysed at all. These data indicate that FusH is a highly specific esterase. The gene is widely distributed among streptomycetes that modify fusidic acid to its inactive lactone derivative.


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