Horizontal transfer - an introduction
This possibility for spread of genes has been generally neglected in the assments of the safety of gene technology by the experts of international organizations like FAO, the Food and Agricultural Organization of United Nations. Still, it will contribute to a very important degree to the spread of genetically engineered genes and some scientists think it may have serious consequences.
Bacteria can take up and spread genes different ways:
1. They can take up naked DNA directly from the surroundings.
Direct uptake of genes from the surroundings is common and widespread. It is called transformation (see Lorencz MG, Wackernagel W 1994). Formerly it was thought that the genes would be broken down by the DNA-digesting enzymes in the environment. But recently it has been found that the DNA is protected from enzyme attacks when adsorbed to solid particles.
In this way genes from genetically engineered organisms may be transferred to bacteria and other microogranisms. The sources may be:
Debris ploughed back from genetically engineered crops. The dead or dying cells are likely to release naked DNA that may survive for many hours. An experiment showed that when adhered to clay soil particles, the DNA survived at an average about 28 hours.
Dead cells from the feces or other excretions of genetically engineered fish may release DNA. Naked DNA may survive on the ocean surface for 45-83 hours and for up to 235 hours in the bottom sediment.
Feces from genetically engineered domestic animals contains myriads of dead and even alive cells with DNA from the animal. Some DNA may have already been transferred bacteria when in the gut.
There is a class of viruses, called bacteriophages that are specialized at infecting bacteria. They can be described as a package of DNA surrounded by a protein coat. This coating has special properties that makes it stick on to bacterial cell walls. When attached, they inject the DNA into the bacteria. Viral DNA has an ability to force the invaded cell to switch over from normal activity to producing millions of copies of the whole virus. Or the virus may insert its DNA into the chromosome of the bacteria.
When new viruses are produced in the bacteria, a piece of bacterial DNA may become packaged in the virus coat. Or a piece of bacterial DNA may become attached to the virus DNA. The virus will thereby carry this bacterial DNA to other bacteria where it may become a part of their chromosome. This is called transduction. Mostly the bacteriophages attach only to a few kinds of bacteria. But they have a tendency to broaden their host range. Bacteriophage transduction is especially common in water. A recent study found as much as 100 million bacteriophages per milliliter in aquatic environments (Bergh O et al 1989)
It has long been believed that mating or conjugation only occurs between members of the same bacterial species. This has recently been contradicted by research that reported a high degree of cross-species mating ("Bacterial Conjugation"). Mating is mediated by a special small piece of DNA, the plasmid that is separate from the bacterial chromosome. Most plasmids are also carriers of antibiotic resistance genes. The plasmids are transferred at mating.
Also a kind of jumping genes are transferred separately at mating. They can jump between different positions in the chromosome or between chromosomes and plasmids. Among these jumping genes there are a special kind of genes that support efficient transfer of antibiotic resistance genes.
Uptake of naked DNA, transfer by virus and transfer by mating all may contribute to uncontrolled spread of genetically engineered genes by microorganisms. The bacteria may spread over wide distances on dust particles and in the sea. They may be ingested by birds who may spread them transcontinentally when they move. The effects are very incompletely known, but it has been suggested that such spread of genes might contribute to the emergence of new bacteria, see Horizontal transfer of viral and bacteria DNA facilitated by GE organisms?.
Jaan Suurküla MD
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