Potential symbiosis-specific genes uncovered by sequencing a 410-kilobase DNA region of the Bradyrhizobium japonicum chromosome

The physical and genetic map of the Bradyrhizobium japonicum chromosome rev ealed that nitrogen fixation and nodulation genes are clustered. Because of the complex interactions between the bacterium and the plant, we expected this chromosomal sector to contain additional genes that are involved in th e maintenance of an efficient symbiosis, Therefore, we determined the nucle otide sequence of a 410-kb region, The overall G+C nucleotide content was 5 9.1%, Using a minimum gene length of 150 nucleotides, 388 open reading fram es (ORFs) were selected as coding regions. Thirty-five percent of the predi cted proteins showed similarity to proteins of rhizobia, Sixteen percent we re similar only to proteins of other bacteria. No database match was found for 29%, Repetitive DNA sequence-derived ORFs accounted for the rest. The s equenced region contained all nitrogen fixation genes and, apart from nodM, all nodulation genes that were known to exist in B, japonicum. We found se veral genes that seem to encode transport systems for ferric citrate, molyb date, or carbon sources. Some of them are preceded by -24/-12 promoter elem ents. A number of putative outer membrane proteins and cell wall-modifying enzymes as well as a type III secretion system might be involved in the int eraction with the host.