New lnu(C) gene conferring resistance to lincomycin by nucleotidylation in Streptococcus agalactiae UCN36

Abstract

Streptococcus agalactiae UCN36 was resistant to lincomycin (MIC = 16 microg/ml) but susceptible to clindamycin (MIC = 0.12 microg/ml) and erythromycin (MIC = 0.06 microg/ml). A 4-kb HindIII fragment was cloned from S. agalactiae UCN36 total DNA on plasmid pUC18 and introduced into Escherichia coli AG100A, where it conferred resistance to lincomycin. The sequence analysis of the fragment showed the presence of a 1,724-bp element delineated by imperfect inverted repeats (22 of 25 bp) and inserted in the operon for capsular synthesis of S. agalactiae UCN36. This element carried two open reading frames (ORF). The deduced amino acid sequence of the upstream ORF displayed similarity with transposases from anaerobes and IS1. The downstream ORF, lnu(C), encoded a 164-amino-acid protein with 26% to 27% identity with the LnuA(N2), LnuA, and LnuA' lincosamide nucleotidyltransferases reported for Bacteroides and Staphylococcus, respectively. Crude lysates of E. coli AG100A containing the cloned lnu(C) gene inactivated lincomycin and clindamycin in the presence of ATP and MgCl2. Mass spectrometry experiments demonstrated that the LnuC enzyme catalyzed adenylylation of lincomycin.

FIG. 1.

Schematic map of the 1,724-bp genetic element bearing the lnu(C) gene. The element contains ORF 1 (black arrow), putatively encoding a homologue of IS1 transposase, and the lnu(C) gene (grey arrow). The 25-bp imperfect inverted repeats (IRL and IRR) are shown, and the mismatches are in bold and underlined. The element is flanked by 8-bp direct repeats.

FIG. 2.

CID-MS2 analysis of the 736.4-amu product resulting from lincomycin modification. (Inset) Proposed structure for adenylyl lincomycin, as deduced from fragments observed after CID. Positions of fragments are indicated by arrows. Note that the ester bond was arbitrarily positioned on C-3 of lincomycin on the figure but could also involve the hydroxyl group of C-2 or that of C-4.