Mobility of DNA
Throughout this book, reference is made to the movement of genes within and between organisms. The reason why it appears at all in a book on environmental biotechnology is to emphasise the ‘oneness’ of the environment, not just at the more obvious level of industrial impact but right down to the interaction between the genetic material of organisms themselves. Plasmids may be transferred between bacteria by conjugation, of which there are several types, but all of which require direct cell to cell contact. Not only are genes transferred between bacteria on plasmids, but bacteriophages (bacterial viruses) are also vectors for intercellular transmission. Similarly, eukaryotic viruses are able to transfer genetic material between susceptible cells. In addition, bacterial cells may pick up DNA free in the environment under conditions where their cell wall has become ‘leaky’ to fragments of this macromolecule, a process called trans-formation. There is also considerable rearrangement of genomic material within an organism stimulated by the presence of transposons. There are many classes of transposable elements which are short pieces of DNA, able to excise themselves, or be excised, out of a genome. Often they take with them neighbouring pieces of DNA, and then reinsert themselves, sometimes with the assistance of other genes, into a second site distinct from the original location on the same genome. Insertion may be into specific sites or random, depending on the nature of the transposon. Transposition normally requires replication of the original DNA frag-ment and so a copy of this transposon is transferred leaving the original behind. Transposition is widespread and occurs in virtually all organisms for which evi-dence of this process has been sought, both prokaryotic and eukaryotic. The term ‘transposable element’, was first coined by Barbara MacClintock, who discovered them in maize, publishing her data in the early 1950s. However, it was not until many years later that the full significance of her work was being recognised, with similar elements being discovered in bacteria. Transposable elements are known to promote the fusion of plasmids within a bacterial cell, where more than one type of plasmid is present. The fusion is stimulated by the presence of insertion sequences (IS), which are short pieces of DNA of a defined and limited range of sequences. They are often found at either end of a transposable element. Their presence enables various DNA rearrangements to take place leading to moder-ation of gene expression. Taking together the reorganisation of DNA within all types of organisms attributable to transposable elements and IS, with transfer of DNA between organisms by plasmids and transformation, in the case of prokary-otes, and viruses in the case of both prokaryotes and eukaryotes, the potential for DNA rearrangement within and between organisms is enormous.
It has been proposed (Reanney 1976), that such transfer is far more univer-sal than had previously been voiced. Transfer of genes by extra chromosomal elements (ECEs), which is the all-embracing name given to include plasmids and viruses, models the means by which molecular evolution takes place in the environment. The proposal is that the evolutionary process occurs principally by insertions and deletions of the genome such as those caused by the activities of ECEs and transposable elements and not by point mutations more frequently observed in isolated cultures such as those maintained in laboratory conditions. It is further suggested that much of the phenotypic novelty seen in evolution is the result of rearrangement of existing structural genes into a different region of the genome and therefore operating under different parameters affecting gene regulation. Transfer of genes across wide taxonomic gaps is made possible by the mobile nature of ECEs many of which may cross species barriers often resulting in the insertion of all or part of the ECE into the recipient genome. Examples of such mobility are viruses which infect a wide host range, such as some retroviruses, the alfalfa mosaic virus, and the Ti plasmid of Agrobacteriumtumefaciens which the bacterium introduces into plant cells. The retroviruses,of which Human Immunodeficiency Virus (HIV) is an example, are unusual in having RNA as their genetic material. They replicate in a manner which includes double-stranded DNA as an intermediate and so may integrate into the host cell genome. RNA viruses tend to be more susceptible than DNA viruses to mutation presumably due to the less chemically stable nature of the macromolecule. They have been invoked by Reanney (1976) as being the likely agents for the spread of genetic information between unrelated eukaryotes. His observations led him to conclude that there is only a blurred distinction between cellular and ECE DNA both in eukaryotes and prokaryotes and further suggest that no organism lives in true genetic isolation as long as it is susceptible to at least one of the classes of ECEs described above. Clearly, for the mutation to be stabilised, it must occur in inheritable DNA sequences, a situation reasonably easy to achieve in microbes and at least possible in multicellular organisms.
The existence of genetic mobility has been accepted for many years, even though the extent and the mechanisms by which it operates are still being elu-cidated. From this knowledge several lessons may be learned; among them, that the genetic environment of any organism may well be significant and that there is some justification in viewing the principle of genetic engineering as perform-ing in the laboratory, a process which is occurring in abundance throughout the living world.