Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Biochemical genetics and the synthesis of nucleic acids and proteins
Genetic methods
Plasmids and episomes

An efficient research method is based on the existence of small genetic elements in Bacteria that exist outside the chromosome. One group of such elements (or factors), designated as F factors, has already been discussed above (Sec. A, 1, c). These elements, which are small circular DNA molecules, belong to a large group of similar agents collectively known as Plasmids and episomes [14, 146, 147]. This group also includes colicinogenic factors and Antibiotic Resistance factors (R factors). Plasmid Replication occurs independently of chromosomal replication, with one or more identical plasmids per bacterial chromosome. Episomes are plasmids capable of integrating into the bacterial chromosome. Some extrachromosomal elements can behave as episomes in some bacteria and as plasmids in others. Plasmids can be infectious (transmissible) or non-infectious. In the former case, they contain genes for the synthesis of sex pili (Sec. A, 1, c) and are capable of transferring their DNA into another Cell. If a plasmid can integrate with the chromosome and subsequently excise from it while carrying along other genes, such a plasmid is called a sex factor. As mentioned above, The process of Gene transfer mediated by the F factor has been widely used in mapping bacterial Chromosomes.

The size of plasmids and episomes can vary. The F-1 sex factor is a circular supercoiled DNA with a Molecular Weight of approximately 62·10⁶. This size is large enough for the factor to contain about 90 genes and have a length of about 30 nm, which accounts for 2.5% of the E. coli chromosome size. Colicinogenic factors [148], which may also be present in E. coli Cells at 10–15 copies per bacterial chromosome, are typically smaller, with a molecular weight of approximately 4·10⁶–5·10⁶. Some larger colicinogenic factors also function as sex factors. These plasmids carry genes that determine the synthesis of toxic protein Antibiotics known as colicins, which are capable of attacking other E. coli strains. The plasmid also carries a gene (or genes) conferring resistance to the toxins in the host bacterial cell. The action of colicin E-3 is that it penetrates a susceptible bacterial cell and inhibits Protein Synthesis by cleaving small fragments from the 3' end of each bacterial 16S rRNA molecule [149] (Fig. 15-14).

Much attention has been paid to antibiotic resistance factors. Due to the widespread use of antibiotics, an important problem arises concerning the rapid development of bacterial resistance to them. This problem can become particularly critical if antibiotics are used indiscriminately, since resistance factors can be readily transferred from one bacterium to another via infectious R-factor plasmids [150–152]. Because the same plasmid can carry resistance genes to A wide variety of antibiotics, "superbugs" resistant to various antibiotics can emerge. The appearance of such bacteria, the probability of which is especially high in hospitals, can lead to serious epidemics of antibiotic-resistant infectious diseases. Resistance mechanisms are often associated with the inactivation of antibiotics. Aminoglycosides, such as streptomycin (Supplement 12-A) and kanamycin, are inactivated by Enzymes that catalyze the phosphorylation or adenylylation of specific hydroxyl groups on the sugar rings. Penicillin (Supplement 7-G) is inactivated by penicillinase, which hydrolytically cleaves the ß-lactam ring. Chloramphenicol (Fig. 14-25) is inactivated by the acylation of one or two hydroxyl groups.

What is THE ORIGIN OF antibiotic resistance factors? Why are genes that ensure the inactivation of such unusual molecules as antibiotics so widespread in nature? Perhaps this is because antibiotic resistance factors normally perform some routine biosynthetic Functions, but the presence of antibiotics in the environment leads to the Selection of mutants whose genes are capable of inactivating them. Nevertheless, it remains unclear why drug-resistance factors appear so frequently specifically in populations treated with antibiotics. A partial solution to the resistance problem has been The Development of semi-synthetic modifications of naturally occurring antibiotics. Since R factors carry genes responsible for the synthesis of enzymes that alter specific Regions of the antibiotic, it is sometimes possible to chemically modify these regions so that they no longer participate in the enzymatic reaction induced by the R factor.

A similarity between infectious Plasmids and Viruses is frequently noted [153]. For example, filamentous Bacteriophages (Supplement 4-B and Fig. 4-8) [154] exit the bacterial cell through the accumulation of hydrophobic protein subunits within the membrane, which form thin microtubules about 6 nm in diameter containing DNA molecules ready for transfer to other bacterial cells. The phage adsorbs to the F pili of a male bacterium, after which the DNA penetrates The Cell via a mechanism that is not yet fully understood [155]. Just as filamentous bacteriophages carry genes encoding their coat protein subunits, F sex factors carry genes responsible for pilin synthesis. Pilin also accumulates in The cell membrane and, as it is extruded, is consumed in the generation of F pili. Thus, it is reasonable to suggest a close relationship between plasmids and viruses. One consequence of DNA transfer from an Hfr strain to F^- bacteria is the penetration of a copy of the F factor into the female bacterium. Because this converts the recipient cell from female to male, Brinton referred to "bacterial sex as a viral disease." A very close similarity also exists between episomes capable of integrating into bacterial chromosomes and "temperate" bacteriophages, which will be discussed in the next section.