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Modern Genetic Analysis Chapter 9. The Genetics of Bacteria and Phages
Bacterial Transformation

In Chapter 2, we learned that the genotype of cells of the bacterium Streptococcus pneumoniae can be altered by the addition of exogenous DNA extracted from a donor culture of a different genotype. The exogenous DNA passes through the cell membrane and can integrate into the bacterial chromosome. This process, generally called transformation, can be demonstrated in many different kinds of bacteria and even in many eukaryotic cells, too. Transformation has some key uses in bacterial genetics (and in eukaryotic genetics).

Bacterial transformation can be demonstrated by using the genes for drug resistance (Figure 9-12). The exogenous transforming DNA is incorporated into the bacterial chromosome by a double-crossover process analogous to that depicted for Hfr × F crosses in Figure 9-6. (However, note that, in conjugation, DNA is transferred from one living cell to another through close contact, whereas, in transformation, isolated pieces of DNA in the external environment are taken up by the cell.)

Transformation can be used to assess linkage. When the DNA from the donor is extracted for use in a transformation experiment, some breakage into smaller pieces is inevitable. If two donor genes are located close together on the chromosome, then there is a greater chance that they will be carried on the same piece of transforming DNA and hence will cause a double transformation. Conversely, if genes are widely separated on the chromosome, then most likely they will be carried on separate transforming segments and the frequency of double transformants will equal the product of the single-gene transformation frequencies. Thus, it should be possible to test for close linkage by testing for a departure from the product rule. Hence, if a+ b+ donor DNA is used to transform a b recipient cells, then, if a and b are closely linked, the proportion of a+ b+ double transformants, or cotransformants, should exceed the product of the proportions of single a+ and b+ transformants. Relative map distances of closely linked genes can be deduced from cotransformation percentages in an approach similar to that used in transduction (considered later in this chapter).

Recombinant DNA technology has now provided the ability to cut DNA at defined positions (by using restriction enzymes; see Chapter 10) and isolate the specific fragments. This technology makes it possible to test various fragments of a bacterial chromosome for the presence of specific genes in transformation tests, and this procedure is also useful for mapping.

MESSAGE
A bacterial genome can be stably transformed into a new genotype by the entry and integration of DNA from another strain. The frequency of cotransformation can be used to decide whether two genes are linked closely together.

© 1999 by W. H. Freeman and Company. All rights reserved.