Abstract
Rhizobium leguminosarum Rlv3841 contains at least three sulfate transporters, i.e., SulABCD, SulP1 and SulP2, and a single molybdate transporter, ModABC. SulABCD is a high-affinity transporter whose mutation prevented growth on a limiting sulfate concentration, while SulP1 and SulP2 appear to be low-affinity sulfate transporters. ModABC is the sole high-affinity molybdate transport system and is essential for growth with NO3(-) as a nitrogen source on limiting levels of molybdate (<0.25 μM). However, at 2.5 μM molybdate, a quadruple mutant with all four transporters inactivated, had the longest lag phase on NO3(-), suggesting these systems all make some contribution to molybdate transport. Growth of Rlv3841 on limiting levels of sulfate increased sulB, sulP1, modB, and sulP2 expression 313.3-, 114.7-, 6.2-, and 4.0-fold, respectively, while molybdate starvation increased only modB expression (three- to 7.5-fold). When grown in high-sulfate but not low-sulfate medium, pea plants inoculated with LMB695 (modB) reduced acetylene at only 14% of the wild-type rate, and this was not further reduced in the quadruple mutant. Overall, while modB is crucial to nitrogen fixation at limiting molybdate levels in the presence of sulfate, there is an unidentified molybdate transporter also capable of sulfate transport.
Highlights
Molybdate transport system and is essential for growth with NO3 as a nitrogen source on limiting levels of molybdate (
We investigated the roles of R. leguminosarum sulfate and molybdate transporters in free-living bacteria and during N-fixing symbiosis on Pisum sativum by analyzing the phenotypes of mutant strains, each lacking one to four of the putative sulfate and molybdate transporter systems identified in the Rlv3841 genome
Bioinformatic analysis of Rlv3841 revealed the presence of a single putative molybdate transporter (ModABC) and three putative sulfate transporters (Fig. 1)
Summary
The triple mutant LMB708 (sulB sulP1 sulP2) had slightly reduced growth compared with the wild-type strain (Fig. 3B) This is consistent with SulP1 and SulP2 being transporters with a lower affinity for sulfate but suggests there are one or more additional unidentified low-affinity S transporters in Rlv3841, as this triple mutant is still able to grow. In. 144 / Molecular Plant-Microbe Interactions contrast to Rlv3841, strain LMB695 (modB)_ failed to grow aerobically in AMS glucose nitrate (Glc/NO3 ) media, nitrate acting as the sole N source, without the addition of molybdate (Fig. 4A). 2.5 μM, the quadruple mutant in which all four Mod and Sul transport systems are inactivated (Table 1, LMB709) had a longer lag phase and slightly reduced growth rate compared with the other mutant strains (Fig. 4D), indicating a role for Sul transporters in molybdate uptake.
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