Evolutionary origin of bacterial chromosome...( Blacksburg Va. Researchers have unv...)

Blacksburg, Va. Researchers have unveiled the evolutionary origin of the different chromosomal architectures found in three species of Agrobacterium. A comprehensive comparison of the Agrobacterium sequence information with the genome sequences of other bacteria suggests a general model for how second chromosomes are formed in bacteria.*

Agrobacteria are members of the Rhizobiaceae family, which also includes the benign, nitrogen-fixing organisms Rhizobium and Sinorhizobium. Agrobacterium tumefaciens C58 is the workhorse of the plant biotechnology science and industry, thanks to its ability to insert its own DNA into host plants.

Members of the Rhizobiaceae have genetic architectures that span single chromosomes, multiple chromosomes and circular DNA molecules (plasmids) of various sizes. The scientists used the sequence information of the genomes of three types of Agrobacterium (biovars), two of which were recently completed, and compared the sequences with those of different bacteria to shed light on the origin of the different chromosomal arrangements.

Most bacteria have only one chromosome. The Rhizobiaceae is an unusual bacterial family in that all of its members have either two chromosomes or one chromosome and very large plasmids. Until this study, it was not clear how such multichromosomal architectures had evolved.

Joo Setubal, associate professor at the Virginia Bioinformatics Institute and the Department of Computer Science at Virginia Tech, commented: "Thanks to the efforts of the Agrobacterium Genome Sequence Consortium and the wider research community, we have sufficient sequence data available from different bacterial species to allow the inference of a general model for bacterial genome evolution. It appears that the transfer of genes from chromosomes to large plasmids mediates second chromosome formation."

"Examination of different genome sequences within the Rhizobiales family has revealed that ge
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