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Trends Microbiol. 2010 Apr;18(4):141-8. doi: 10.1016/j.tim.2009.12.010. Epub 2010 Jan 18.

Introducing the bacterial 'chromid': not a chromosome, not a plasmid.

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Department of Biology, University of York, PO Box 373, York YO10 5YW, UK.


In addition to the main chromosome, approximately one in ten bacterial genomes have a 'second chromosome' or 'megaplasmid'. Here, we propose that these represent a single class of elements that have a distinct and consistent set of properties, and suggest the term 'chromid' to distinguish them from both chromosomes and plasmids. Chromids carry some core genes, and their nucleotide composition and codon usage are very similar to those of the chromosomes they are associated with. By contrast, they have plasmid replication and partitioning systems and the majority of their genes confer accessory functions. Chromids seem particularly rich in genus-specific genes and appear to be 'reinvented' at the origin of a new genus.

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