The nodules lacked lenticels and fixed two times less nitrogen (Rojas-Jiménez et al., 2005). The three R. tropici mutants (ΔolsC, ΔolsE, and ΔolsCΔolsE) lacking OL hydroxylases established their symbiosis only poorly (Vences-Guzmán et al., 2011). As R. tropici is challenged by low pH conditions inside its host plant (Udvardi et al., 1991; Udvardi & Day, 1997), it can be speculated that the observed symbiotic phenotype

is a consequence of the mutants’ increased acid sensitivity. The OL hydroxylase OlsD was first isolated from B. cenocepacia J2315, a β-proteobacterium known as an opportunistic pathogen of humans. González-Silva et al. (2011) originally suggested that 2-hydroxylation of OLs in B. cenocepacia might be performed by an LpxO homolog called OlsD (BCAM2401). OlsD indeed hydroxylated

OL, but the hydroxylation did not occur on the ester-linked fatty acid. Surprisingly, Cyclopamine solubility dmso data obtained by mass spectrometry suggested that OlsD modifies the amide-linked fatty acid of OLs with a hydroxyl group (Fig. 2), a modification that was previously unknown. Unfortunately, their analysis did not allow for the determination of the exact position of the hydroxyl group. OlsD from B. cenocepacia is a 249-amino-acid protein, apparently lacking transmembrane helices (González-Silva et al., 2011). It is widely distributed within the genus Burkholderia, but homologs are also present in three Serratia strains. The gene coding for the 2-hydroxylase see more activity hydroxylating the ester-linked fatty acyl residue in the C-2 position in B. cenocepacia has not been identified yet. Rojas-Jiménez et al. (2005) had described the presence of four different OLs in R. tropici CIAT899. The presence of OlsC alone could not explain this number of distinct structures. Using a functional expression screen conjugating a cosmid bank from R. tropici into S. meliloti, Vences-Guzmán et al. (2011) identified the gene olsE coding for the hydroxylase OlsE. Mass spectrometry analysis showed that OlsE introduced a hydroxyl group in the ornithine moiety. So far,

the exact position of the hydroxylation could not be determined, but ninhydrin staining of the VAV2 different OLs shows that the hydroxyl group affects the reactivity of the lipid to ninhydrin. Bioinformatic predictions indicate that the OlsE protein (331 amino acids) from R. tropici CIAT899 is highly hydrophobic and might form between 4 and 6 transmembrane helices. OlsE belongs to the fatty acyl hydroxylase superfamily (cl01132), which is characterized by the presence of two copies of the HXHH motif. This superfamily includes fatty acid and carotene hydroxylases, sterol desaturases, and C-4 sterol methyl oxidase (Arthington et al., 1991; Bard et al., 1996; Mitchell & Martin, 1997; Kennedy et al., 2000). A similar motif can be found in membrane-bound fatty acid desaturases such as OLE1 from Saccharomyces cerevisiae and in bacterial alkane hydroxylase and xylene monooxygenase (Kok et al.