Figure 2: Expression of active ErmC decreases the fitness of S. aureus.

(a) Principle of detection of A2058 dimethylation by primer extension. In the presence of dATP, dGTP, dTTP and ddCTP, reverse transcriptase extends the 18-nucleotide (nt) long primer by four nucleotides when A2058 is not modified but only by 2 nucleotides when the residue is dimethylated, because extension is impeded by the modification. The 20-nt band on the gel reflects the extent of dimethylation of A2058 in rRNA from S. aureus USA300 cells containing pLI50, pErmCC or pErmCC* plasmids (lanes 2–4, respectively). Lane 1 contains the labelled primer. (b,c) Growth competition between S. aureus USA300 cells transformed with plasmids pErmCC and pLI50 (b), or pErmCC and pErmCC* (c). The percentage of cells expressing active ErmC was estimated by measuring the level of A2058 methylation every 10 generations for a total of ca. 40 generations. Values are the mean of three independent experiments; error bars indicate the s.d. of the mean.