Novel mechanisms of resistance to lincosamides in Staphylococcus and Arthrobacter spp

A new evolutionary variant of the streptogramin A resistance protein, Vga(A)LC, from Staphylococcus haemolyticus with shifted substrate specificity towards lincosamides

We found a new variant of the streptogramin A resistance gene, vga(A)LC, in clinical isolates of Staphylococcus haemolyticus resistant to lincomycin and clindamycin but susceptible to erythromycin and in which no relevant lincosamide resistance gene was detected. The gene vga(A)LC, differing from the gene vga(A) at the protein level by seven amino acid substitutions, was present exclusively in S. haemolyticus strains resistant to both lincosamides and streptogramin A (LS(A) phenotype). Antibiotic resistance profiles of the ATP-binding cassette (ABC) proteins Vga(A)(LC) and Vga(A) in the antibiotic-susceptible host S. aureus RN4220 were compared. It was shown that Vga(A)LC conferred resistance to both lincosamides and streptogramin A, while Vga(A) conferred significant resistance to streptogramin A only. Detailed analysis of the seven amino acid substitutions, distinguishing the two related ABC proteins with different substrate specificities, identified the substrate-recognizing site: four clustered substitutions (L212S, G219V, A220T, and G226S) in the spacer between the two ATP-binding cassettes altered the substrate specificity and constituted the lincosamide-streptogramin A resistance phenotype. A transport experiment with radiolabeled lincomycin demonstrated that the mechanism of lincosamide resistance in S. haemolyticus was identical to that of the reported macrolide-streptogramin B resistance conferred by Msr(A).

An ATP-binding cassette transporter and two rRNA methyltransferases are involved in resistance to avilamycin in the producer organism Streptomyces viridochromogenes Tü57

Three different resistance factors from the avilamycin biosynthetic gene cluster of Streptomyces viridochromogenes Tü57, which confer avilamycin resistance when expressed in Streptomyces lividans TK66, were isolated. Analysis of the deduced amino acid sequences showed that AviABC1 is similar to a large family of ATP-binding transporter proteins and that AviABC2 resembles hydrophobic transmembrane proteins known to act jointly with the ATP-binding proteins. The deduced amino acid sequence of aviRb showed similarity to those of other rRNA methyltransferases, and AviRa did not resemble any protein in the databases. Independent expression in S. lividans TK66 of aviABC1 plus aviABC2, aviRa, or aviRb conferred different levels of resistance to avilamycin: 5, 10, or 250 microg/ml, respectively. When either aviRa plus aviRb or aviRa plus aviRb plus aviABC1 plus aviABC2 was coexpressed in S. lividans TK66, avilamycin resistance levels reached more than 250 microg/ml. Avilamycin A inhibited poly(U)-directed polyphenylalanine synthesis in an in vitro system using ribosomes of S. lividans TK66(pUWL201) (GWO), S. lividans TK66(pUWL201-Ra) (GWRa), or S. lividans TK66(pUWL201-Rb) (GWRb), whereas ribosomes of S. lividans TK66 containing pUWL201-Ra+Rb (GWRaRb) were highly resistant. aviRa and aviRb were expressed in Escherichia coli, and both enzymes were purified as fusion proteins to near homogeneity. Both enzymes showed rRNA methyltransferase activity using a mixture of 16S and 23S rRNAs from E. coli as the substrate. Coincubation experiments revealed that the enzymes methylate different positions of rRNA.