Inability To Catabolize Galactose Leads to Increased Ability To Compete for Nodule Occupancy in Sinorhizobium meliloti

Abstract

ABSTRACTA mutant unable to utilize galactose was isolated inSinorhizobium melilotistrain Rm1021. The mutation was found to be in a gene annotateddgoK1, a putative 2-keto-3-deoxygalactonokinase. The genetic region was isolated on a complementing cosmid and subsequently characterized. Based on genetic and bioinformatic evidence, the locus encodes all five enzymes (galD,dgoK,dgoA,SMc00883, andilvD1) involved in the De Ley-Doudoroff pathway for galactose catabolism. Although all five genes are present, genetic analysis suggests that the galactonase (SMc00883) and the dehydratase (ilvD1) are dispensable with respect to the ability to catabolize galactose. In addition, we show that the transport of galactose is partially facilitated by the arabinose transporter (AraABC) and that both glucose and galactose compete with arabinose for transport. Quantitative reverse transcription-PCR (qRT-PCR) data show that in adgoKbackground, the galactose locus is constitutively expressed, and the induction of thearalocus seems to be enhanced. Assays of competition for nodule occupancy show that the inability to catabolize galactose is correlated with an increased ability to compete for nodule occupancy.