Department of Biology, University of York, PO Box 373, York YO10 5YW, UK.
Trends Microbiol. 2010 Apr;18(4):141-8. doi: 10.1016/j.tim.2009.12.010. Epub 2010 Jan 18.
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.
除了主要染色体外，大约十分之一的细菌基因组还有一个“第二染色体”或“大质粒”。在这里，我们提出这些代表了一类具有独特而一致的属性的元素，并建议使用“chromid”这个术语来将它们与染色体和质粒区分开来。chromids 携带一些核心基因，其核苷酸组成和密码子使用与它们相关的染色体非常相似。相比之下，它们具有质粒复制和分区系统，并且它们的大多数基因赋予辅助功能。chromids 似乎特别富含属特异性基因，并且似乎在新属的起源处被“重新发明”。
