Variation in genome size and content is common among bacterial strains. Identifying these naturally occurring differences can accelerate our understanding of bacterial attributes, such as ecological specialization and genome evolution. In this study, we used representational difference analysis to identify potentially novel sequences not present in the sequenced laboratory strain Rm1021 of the nitrogen-fixing bacterium
Sinorhizobium meliloti. Using strain Rm1021 as the driver and the type strain of S. melilotiATCC 9930, which has a genome size ∼370 kilobases bigger than that of strain Rm1021, as the tester, we identified several groups of sequences in the ATCC 9930 genome not present in strain Rm1021. Among the 85 novel DNA fragments examined, 55 showed no obvious homologs anywhere in the public databases. Of the remaining 30 sequences, 24 contained homologs to the Rm1021 genome as well as unique segments not found in Rm1021, 3 contained sequences homologous to those published for another S. melilotistrain but absent in Rm1021, 2 contained sequences homologous to other symbiotic nitrogen-fixing bacteria ( Rhizobium etliand Bradyrhizobium japonicum), and 1 contained a sequence homologous to a gene in a non-nitrogen-fixing species, Pseudomonassp. NK87. Using PCR, we assayed the distribution of 12 of the above 85 novel sequences in a collection of 59 natural S. melilotistrains. The distribution varied widely among the 12 novel DNA fragments, from 1.7% to 72.9%. No apparent correlation was found between the distribution of these novel DNA sequences and their genotypes obtained using multilocus enzyme electrophoresis. Our results suggest potentially high rates of gene gain and loss in S. melilotigenomes.