The calcium-stimulated lipid A 3-O deacylase from Rhizobium etli is not essential for plant nodulation

Abstract

The lipid A component of lipopolysaccharide from the nitrogen-fixing plant endosymbiont, Rhizobium etli, is structurally very different from that found in most enteric bacteria. The lipid A from free-living R. etli is structurally heterogeneous and exists as a mixture of species which are either pentaacylated or tetraacylated. In contrast, the lipid A from R. etli bacteroids is reported to consist exclusively of tetraacylated lipid A species. The tetraacylated lipid A species in both cases lack a β-hydroxymyristoyl chain at the 3-position of lipid A. Here, we show that the lipid A modification enzyme responsible for 3-O deacylation in R. etli is a homolog of the PagL protein originally described in Salmonella enterica sv. Typhimurium. In contrast to the PagL proteins described from other species, R. etli PagL displays a calcium dependency. To determine the importance of the lipid A modification catalyzed by PagL, we isolated and characterized a R. etli mutant deficient in the pagL gene. Mass spectrometric analysis confirmed that the mutant strain was exclusively tetraacylated and radiochemical analysis revealed that 3-O deacylase activity was absent in membranes prepared from the mutant. The R. etli mutant was not impaired in its ability to form nitrogen-fixing nodules on Phaseolus vulgaris but it displayed slower nodulation kinetics relative to the wild-type strain. The lipid A modification catalyzed by R. etli PagL, therefore, is not required for nodulation but may play other roles such as protecting bacterial endosymbionts from plant immune responses during infection.

Structural and biochemical characterization of a carbohydrate acetylesterase from Sinorhizobium meliloti 1021

“…Interestingly, many plant‐associated microorganisms have developed carbohydrate acetylesterases to remove the acetyl groups from plant carbohydrates [6]. Specifically, the genus Rhizobia regulate symbiotic relationships with legume plants through the actions of carbohydrate acetylesterases and glycoside hydrolases [7–9]. The resulting deacetylated carbohydrates are important signaling molecules for the initiation of nodule formation on host legume roots and symbiotic fixation of nitrogen by Rhizobia species.…”