Demonstration of Extrachromosomal Elements
Mycoplasmas are parasites and pathogens of plants, insects, and animals, including humans. Although mycoplasmas are highly evolved parasites, they are not immune to being parasitized themselves. Within the class Mollicutes, extrachromosomal elements have been described for several genera, but although abundant in the spiroplasmas, are generally very rare (1,2). The plasmids identified so far are cryptic and have come from only two species, Mycoplasma mycoides subsp. mycoides and Spiroplasma citri (3). The best characterized are <2 kb in size, and contain genes only necessary for plasmid replication and maintenance. Sequence analysis indicates that they are related to a large family of Gram-positive bacterial plasmids that replicate by way of single-stranded DNA intermediates (4).
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