SUMMARY: Inhibition studies with free-living cells of 2011 showed that succinate, fumarate and malate are transported via a common transport system. It is an active process that is inducible by succinate. The apparent for succinate uptake in free-living cells is 5·3 μ. Seven Tn5-induced mutants that did not grow on succinate, fumarate or malate lacked the C-dicarboxylate transport system in the free-living state (Detfl). Five of these mutants (RMS11, RMS16, RMS17, RMS24, RMS118) induced nodules on alfalfa with about half the nitrogen-fixation activity of the wild-type (Fix). The remaining two mutants (RMS420 and RMS938) gave rise to small, white and ineffective nodules on alfalfa (Fix). These results correlated with the dicarboxylate transport rates of bacteroids isolated from appropriate nodules: bacteroids isolated from nodules induced by the Fix mutants transported succinate or malate at about 30-50% of the wild-type rate (Dcts), whereas bacteroids isolated from nodules induced by Fix-mutants showed no uptake activity (Dcts). From an 2011 bank three cosmids were identified which complemented different Dctfl mutants. Complemented Dctfl mutants could grow again on agar containing succinate or malate. In the case of Fix mutants complementation resulted in effective nodules. The cosmids pRmSC101 and pRmSC102 complemented the Fix mutant RMS420. A third cosmid, pRmSC121, complemented the second Fix mutant RMS938 and nearly all of the Fix mutants. For the Fix mutant RMS420 the Tn5 insertion site was found to be located on a 15·3 RI fragment which is also part of the complementing cosmid pRmSC102.


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