Mutation in the pssA gene involved in exopolysaccharide synthesis leads to several physiological and symbiotic defects in Rhizobium leguminosarum bv. trifolii.

Monika Janczarek,Kamila Rachwał

doi:10.3390/ijms141223711

Monika Janczarek, Kamila Rachwał

Open Access

https://doi.org/10.3390/ijms141223711

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Abstract

The symbiotic nitrogen-fixing bacterium Rhizobium leguminosarum bv. trifolii 24.2 secretes large amounts of acidic exopolysaccharide (EPS), which plays a crucial role in establishment of effective symbiosis with clover. The biosynthesis of this heteropolymer is conducted by a multi-enzymatic complex located in the bacterial inner membrane. PssA protein, responsible for the addition of glucose-1-phosphate to a polyprenyl phosphate carrier, is involved in the first step of EPS synthesis. In this work, we characterize R. leguminosarum bv. trifolii strain Rt270 containing a mini-Tn5 transposon insertion located in the 3′-end of the pssA gene. It has been established that a mutation in this gene causes a pleiotropic effect in rhizobial cells. This is confirmed by the phenotype of the mutant strain Rt270, which exhibits several physiological and symbiotic defects such as a deficiency in EPS synthesis, decreased motility and utilization of some nutrients, decreased sensitivity to several antibiotics, an altered extracellular protein profile, and failed host plant infection. The data of this study indicate that the protein product of the pssA gene is not only involved in EPS synthesis, but also required for proper functioning of Rhizobium leguminosarum bv. trifolii cells.

Highlights

Rhizobium leguminosarum bv. trifolii is a gram-negative bacterium that can exist as either a free-living bacterium or a nitrogen-fixing symbiont inside root nodules of its host plant—clover (Trifolium pratense)
Previous studies of other researchers indicated that the pssA gene encodes a protein of the length of 200 amino acids, which is located in the bacterial inner membrane and involved in the first step of extracellular polysaccharide (EPS) synthesis [7,11,14]
Our earlier data showed that the pssA gene represents an individual open reading frame located downstream of pssB and is present in genomes of all strains belonging to three R. leguminosarum biovars (Figure 1) [23]

Summary

Introduction

Rhizobium leguminosarum bv. trifolii is a gram-negative bacterium that can exist as either a free-living bacterium or a nitrogen-fixing symbiont inside root nodules of its host plant—clover (Trifolium pratense). Trifolii is a gram-negative bacterium that can exist as either a free-living bacterium or a nitrogen-fixing symbiont inside root nodules of its host plant—clover (Trifolium pratense). This symbiosis is a complex process requiring the exchange of signaling molecules between both partners. Host plant roots secrete flavonoids that induce the bacterium to produce the Nod factor, the primary determinant for formation of nodules, i.e., specialized new structures within which nitrogen fixation occurs [1]. After infection of nodule cells, bacteria differentiate into symbiotic forms called bacteroids, which reduce atmospheric nitrogen to ammonia utilized by the host plant [2].

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