Whereas increased trehalose levels in R. etli inoculant strains seem to favor drought Q VD Oph tolerance of the host legume, the involvement of trehalose in desiccation tolerance of R. etli free-living cells has not been investigated. In this work, we address the role

of trehalose in heat and desiccation tolerance of this soil bacterium. Based on genome analysis, we reconstructed the R. etli trehalose metabolism, and found evidence for a horizontal transfer origin of the otsA copy located in plasmid p42a. In addition, we showed that inactivation of the chromosomal copy Fosbretabulin of otsA (otsAch) completely abolishes trehalose synthesis by R. etli in mannitol minimal medium. Finally, we showed an important role for trehalose in thermoprotection and desiccation

tolerance of R. etli free-living cells. Methods Bacterial strains, plasmids and culture conditions The bacterial strains and plasmids used are listed in Table 1. R. etli CE3 (a spontaneous Smr mutant of R. etli CFN 42T) [31] was used as the wild type strain. R etli strains were routinely grown in complex TY medium [32]. E. coli strains were grown aerobically in complex LB medium [33]. B-medium [34], which contains 10 g l-1mannitol as the sole carbon source, was used as minimal medium for R. etli. When appropriate, trehalose and glucose were also used as carbon source at a final concentration of 20 mM. Osmotic strength of STAT inhibitor this medium was increased by the addition of 0.1 to 0.2 M final concentration of NaCl. pH was adjusted to 7.2 (for TY) or 5 (for B-). Solid media contained 2% of Bacto agar (Difco). E. coli cultures were incubated at 37°C. R. etli cultures were incubated at 28°C or 35°C [29]. When used, filter sterilized antibiotics were added at the following final concentrations (μg ml-1): ID-8 ampicillin (ap), 150 for E. coli; chloramphenicol, 30 for E. coli; gentamicin (Gm), 20 for E. coli, 25 for R. etli; streptomycin (Sm) 20 for E. coli, 40 for R. etli; spectinomycin (Spc) 80–100

for R. etli and nalidixic acid 20 for R. etli. When appropriate, the following compounds were added to the media (final concentration): X-Gal (5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside, Sigma, 40 μg/ml), IPTG (isopropyl-β-D-1-thiogalactopyranoside, Sigma, 25 μg/ml). Growth was monitored as the optical density of the culture at 600 nm (OD600) with a Perkin-Elmer Lambda 25 UV/Vis spectrophotometer. Table 1 Bacterial strains and plasmids used in this study Strain or plasmid Relevant genotype and/or description Source or reference R. etli strains      CFN 42 Wild type [29]  CE3 Spontaneous Smr mutant of R, etli CFN 42 Nalr [31]  CMS310 R. etli CE3 otsAch::ΩNalrSmrSpcr This study E.