(A) Analysis of luxS expression in RPMI medium. Northern blot analysis of total RNA isolated from RDN02 (wild-type M19 strain) cultures at early-logarithmic (EL), mid-logarithmic (ML) and late-logarithmic (LL) phase of growth. Blots were hybridized with probes specific to luxS with 5S rRNA as loading control. The estimated sizes of transcripts are indicated. The blots shown are representative of three independent experiments. Identical luxS expression profiles were observed with RDN29 (wild-type M1 strain). (B) Induction of luminescence in V. harveyi reporter strain BB170 by CM of S. pyogenes. RDN02: M19 wild-type, RDN306: isogenic luxS-deficient mutant. RPMI medium and CM of DH5-α (AI-2 deficient) were used as negative controls; CM of BB170 was used as positive control. CM was prepared from cultures at late-logarithmic phase of growth in RPMI medium. The luminescence values expressed in specific light units (SLU = RLU (relative light units)/OD620 nm) (y axis) are plotted versus time (h) (x axis). Data shown are representative of three independent experiments. Similar data were obtained with CM of derivative strains of the RDN29 background.