The dnaA gene of Rhizobium meliloti lies within an unusual gene arrangement

Abstract

Rhizobium meliloti exists either as a free-living soil organism or as a differentiated endosymbiont bacteroid form within the nodules of its host plant, alfalfa (Medicago sativa), where it fixes atmospheric N2. Differentiation is accompanied by major changes in DNA replication and cell division. In addition, R. meliloti harbors three unique large circular chromosome-like elements whose replication coordination may be complex. As part of a study of DNA replication control in R. meliloti, we isolated a dnaA homolog. The deduced open reading frame predicts a protein of 57 kDa that is 36% identical to the DnaA protein of Escherichia coli, and the predicted protein was confirmed by immunoblot analysis. In a comparison with the other known DnaA proteins, this protein showed the highest similarity to that of Caulobacter crescentus and was divergent in some domains that are highly conserved in other unrelated species. The dnaA genes of a diverse group of bacteria are adjacent to a common set of genes. Surprisingly, analysis of the DNA sequence flanking dnaA revealed none of these genes, except for an rpsT homolog, also found upstream of dnaA in C. crescentus. Instead, upstream of rpsT lie homologs of fpg, encoding a DNA glycosylase, and fadB1, encoding an enoyl-coenzyme A hydratase with a strikingly high (53 to 55%) level of predicted amino acid identity to two mammalian mitochondrial homologs. Downstream of dnaA, there are two open reading frames that are probably expressed but are not highly similar to any genes in the databases. These results show that R. meliloti dnaA is located within a novel gene arrangement.