BRADYRHIZOBIUM-JAPONICUM DOES NOT REQUIRE ALPHA-KETOGLUTARATE DEHYDROGENASE FOR GROWTH ON SUCCINATE OR MALATE

The sucA gene, encoding the El component of alpha-ketoglutarate dehydr ogenase, was cloned from Bradyrhizobium japonicum USDA110, and its nuc leotide sequence was determined, The gene shows a codon usage bias typ ical of non-nif and non-fix genes from this bacterium, with 89.1% of t he codons being G or C in the third position. ii mutant strain of B, j aponicum, LSG184, was constructed with the sucA gene interrupted by a kanamycin resistance marker, LSG184 is devoid of alpha-ketoglutarate d ehydrogenase activity, indicating that there is only one copy of sucA in B. japonicum and that it is completely inactivated in the mutant, B atch culture experiments on minimal medium revealed that LSG184 grows well on a variety of carbon substrates, including arabinose, malate, s uccinate, beta-hydroxybutyrate, glycerol, formate, and galactose, The sucA mutant is not a succinate auxotroph but has a reduced ability to use glutamate as a carbon or nitrogen source and an increased sensitiv ity to growth inhibition by acetate, relative to the parental strain, Because LSG184 grows well on malate or succinate as its sole carbon so urce, we conclude that B. japonicum, unlike most other bacteria, does not require an intact tricarboxylic acid (TCA) cycle to meet its energ y needs when growing on the four-carbon TCA cycle intermediates, Our d ata support the idea that B, japonicum has alternate energy-yielding p athways that could potentially compensate for inhibition of alpha-keto glutarate dehydrogenase during symbiotic nitrogen fixation under oxyge n-limiting conditions.