, 2009). Mesorhizobium loti induced small white ‘tumor-like’ growth on Leucaena leucocephala, but a mutant in a conserved structural component of T3SS (the HDAC phosphorylation M. loti rhcJ mutant strain) formed large pink nodules (Hubber et al., 2004). Little is known about the role of each of the putative M. loti T3SS effectors. The protein encoded by mlr6316 has been described to have a partial negative
effect on Le. leucocephala nodulation (Hubber et al., 2004), whereas the protein encoded by mlr6361 has been described to have a negative role in Lo. halophilus nodulation (Okazaki et al., 2010). However, it has not yet been determined which of the proteins secreted by the M. loti T3SS are involved in the positive nodulation effects observed in some Lotus spp. The aim of this work
was to determine whether the N-terminal regions of proteins encoded by mlr6316 and mlr6331 are able to direct the protein secretion via M. loti T3SS and to determine the involvement of the different T3SS putative effectors in the symbiosis with two different Lotus species. Bacterial strains and plasmids used in this study are listed in Supporting Information, Table S1. MAFF303099 strains were grown at 28 °C in AB minimal medium (Douglas et al., 1985) supplemented with sucrose (0.5% w/v). When necessary, antibiotics were added to the following final concentrations (μg mL−1): gentamicin (Gm), 30; ampicillin (Amp), 100; neomycin GSI-IX (Nm), 100; spectinomycin (Sp), 100; and tetracycline (Tc), 10 for Escherichia coli or 1 for M. loti. For T3SS induction, naringenin was added to cultures at an optical density 600 nm (OD600 nm) of 0.1 to a final concentration of 1 μM. pBAD-mlr6331 was constructed as previously described (Sánchez et al., 2009). The oligonucleotide primer pairs used are described in Table S1. Sequences encoding the N-terminal portion of the protein, together with the upstream
promoter region, were cut from pBAD-mlr6361 (59 aa), pBAD-mlr6316 (160 aa), pBAD-mlr6358 (160 aa) (Sánchez et al., 2009), and pBAD-mlr6331 (177 aa), respectively, and cloned into the pK18mobTc vector (Sánchez et al., 2009). The same plasmids were also Rapamycin solubility dmso introduced by biparental mating into an M. loti rhcN pMP2112 mutant strain. Supernatant protein extractions were carried out by direct TCA precipitation as previously described (Sánchez et al., 2009). Supernatant was concentrated approximately 2000 times. For total (intracellular) bacterial protein extractions, 1 ml of the bacterial cultures used above was centrifuged and the pellets dissolved in cracking buffer. Proteins were separated using SDS-PAGE and then stained using silver nitrate. For immunoblotting, anti-NGR234 strain NopA (Marie et al., 2004) or a commercially available anti-FLAG M2 monoclonal antibody (Sigma) was used. Analyzed mutants and oligonucleotides pairs used for their construction are described in Table S1.