Arrangement of nodulation genes (A), nitrogen fixation genes (B), and gene clusters in the 7653R plasmid pMhu7653Rb (C). Double slash marks represent genome regions that are not shown. In the clusters, nitrogen-fixation genes (B) conserved among the six strains are represented by white arrows, while those varying in copy number, location, or transcriptional orientation are shown in different colors. Nodulation and nitrogen-fixation gene clusters (C) in 7653R plasmid pMhu7653Rb. Genes are colored according to their names. Double slash marks represent genome regions that are not shown. Arrows indicate the location of potential nod-boxes.