The bacterial strains and plasmids used in this study are listed in Table 1. Wild-type S. meliloti strain Rm8530 is identical to strain 1021 except that it has a functional copy of the transcriptional regulator gene expR, which is required for QS [20 (link)]. PY (peptone-yeast extract) and LB (Luria broth) complex media and MMSN (minimal medium succinate ammonium) were described previously [7 (link)] and solidified with 1.5 % agar when necessary. Bromfield medium containing 0.5 % or 0.3 % Difco Noble Agar (Beckman, Dickinson and Co., Sparks, MD, USA) were prepared as described by Bahlawane [21 (link)]. Putrescine ·2HCl [Put; H2N(CH2)4NH2], cadaverine [Cad; H2N(CH2)5NH2], spermine [Spm; H2N(CH2)3NH(CH2)4NH(CH2)3NH2], spermidine [Spd; H2N(CH2)3NH(CH2)4NH2], 1,3-diaminopropane [DAP; H2N(CH2)3NH2] and norspermidine [NSpd; H2N(CH2)3NH(CH2)3NH2] were purchased from Sigma (St. Louis, MO, USA) and homospermidine·3HCl [HSpd; H2N(CH2)4NH(CH2)4NH2] was obtained from Santa Cruz Biotechnology (Santa Cruz, CA, USA). Aqueous 200 mM PA stock solutions were adjusted to pH 6.8, filter sterilized and added to cultures to a final concentration of 0.1 mM. When required, antibiotics were used at the following concentrations (µg ml−1): gentamicin (Gm), 15; kanamycin (Km), 50; spectinomycin (Sp), 100; and streptomycin (Sm), 200.
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