Extrachromosomal elements

PAC A high capacity (70-95 kb) cloning vector based upon the lytic E. colt bacteriophage PI that replicates in bacteria as an extrachromosomal element. 

A line not present in the example is the OG (OrGanelle) line. This line is used to indicate in what organelle or extrachromosomal element the gene is encoded. Example ... 

The small pieces of DNA known as plasmids, which replicate independently of the chromosomes, have been discussed briefly in Chapter 5. Plasmids share a number of properties with viruses, and both are important to the techniques of contemporary molecular biology and genetic engineering. Bacterial plasmids may be present as one or several copies for each chromosome. Episomes are plasmids that are able to become integrated into the bacterial chromosome. Some extrachromosomal elements are episomes in one host and plasmids in another. Bacterial... 

Because the jS-lactamase concerned is, in this instance, carried by an extrachromosomal element, it is possible to obtain R variants of the lactamase-producing strain. The availability of the R variants allows the effect of /3-lactamase production on the resistance of the pseudomonas strains to be studied directly. Table 7.8 compares single-cell resistance values obtained with R and R variants of a strain of Ps. aeruginosa. The presence of the enzyme (expressed in the R strain at a level of 3 units/mg dry wt. bacteria) increases the resistance more than 100-fold in the case of carbenicillin and penicillin G, and more than 10-fold for cephaloridine and ampicillin. 

Since food enterococci do not seem to be associated with the potentially high virulence profiles, when compared with clinical strains, it seems that their presence in traditional cheeses results in a low health risk (Eaton and Gasson 2001 Semedo et al. 2003). Nevertheless, the interaction of such strains with susceptible hosts seems to be a concern mainly in high-risk population groups, i.e. immunocompromised patients. These findings, associated with the natural ability of enterococci to acquire, accumulate and share extrachromosomal elements within their genus and with other bacteria, require further studies to evaluate the consequent risk for human health. The question of whether enterococci possessing virulence determinants present in cheese are safe remains a question. However, Bryndza cheese has been consumed safely for more than 200 years and was never associated with foodbome diseases (Kerestes and Selecky 2003). 

The exact mechanisms for microbial adaptation to the pesticide molecule in soils that develop enhanced degradation capacity are not completely understood. These processes could be viewed from the ecological and population aspects, from their biochemical and enzymatic reactions, or from the genetic aspects, in which extrachromosomal elements may be involved as part of the process. 

The circular and linear extrachromosomal elements in Leishmania can either be maintained or lost in the absence of selective pressure exerted by the corresponding drug. It has been conjectured that the formation and maintenance of extrachromosomal circles is stabilized only after prolonged exposure of cells to drug. However, after two years of maintenance under selective pressure, the linear DNA molecules still retain their inherent stability (MPA resistance) or instability (DFMO) resistance. The cis acting elements that provide centromeric and telomeric function to leishmanial DNAs have not yet been elucidated. 

Hanson, S., Beverly, S. M., Wagner, W. and Ullman, B. (1992) Unstable amplification of two extrachromosomal elements in a-difluoromethylornithine resistant Leishmania donovani. Mol. Cell. Biol. 12 5499-5507. 

Appendix C-V. Extrachromosomal Elements of Gram Positive Organisms... 

Recombinant DNA molecules derived entirely from extrachromosomal elements of the organisms listed below (including shuttle vectors constructed from vectors described in Appendix C), propagated and maintained in organisms listed below are exempt from these NIH Guidelines. 

Chiscon and Kohne (1969) have shown, by repeated fractionation with hydroxyapatite, that there is a fraction, other than rDNA, of repeated nucleotide sequences in coli. Chiscon and Kohne conclude that the repeated nucleotide sequences are in some extrachromosomal element, presumably a plasmid. An analogous situation occurs in the eukaryotes when the presence of multiple copies of mitochondrial DNA within a single ceU might be considered as repetitious DNA. However, in this article we shall only consider repetitious nucleotide sequences that are an integral part of nuclear DNA. 

Plasmids are circular genetic, extrachromosomal, elements. There are one to a few dozen copies per bacteria. They are especially designed as vectors for the gene amplification. Many plasmids carry genes of resistance to antibiotics. 

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