How does transformation occur

The uptake of foreign DNA from the environment is known to occur naturally in a number of bacterial types, both Gram-positive and Gram-negative, by taking up fragments of naked DNA released from dead cells in the vicinity. Being linear and very fragile, the DNA is easily broken into fragments, each carrying on average around 10 genes. Transformation will only happen at a specific stage in the bacterial life cycle, when cells are in a physiological state known as competence. This occurs at different times in different bacteria, but is commonly during late log phase. One of the reasons why only a low percentage of recipient cells become transformed is that only a small proportion of them are at any one time in a state of competence. The expression of proteins essential to the transformation process is dependant on the secretion of a competence factor.

The exact mechanism of transformation varies somewhat according to species; the process for Bacillus subtilis is shown in Figure 11.25. Mere uptake of exogenous DNA is not enough to cause transformation; it must also be integrated into the host genome, displacing a single strand, which is subsequently degraded. Upon DNA replication and cell division, one daughter cell will inherit the parent genotype, and the other

Box 11.7 Transformation is not due to reverse mutation

At the time of Griffith's experiment, it was already known that the wildtype S-form could mutate to the R-form and vice versa. It might therefore be argued that the results of his fourth experiment could easily be explained away in this way. Griffith's experiment was sufficiently well designed to refute this argument, however, because the bacteria he used were of two different serotypes, meaning that they produced different types of capsule (types II and III). Even if the IIR-form cells had mutated back to the wildtype (IIS), they could not have produced the type III capsule Griffith observed in the bacteria he recovered from the mice. The only explanation is that the ability to make type III capsules had been acquired from the heat-killed type III cells.

Degraded second strand

Degraded second strand b)

Recombination with homologous sequence

Recombination with homologous sequence c)

Replacement of host strand by donor DNA

Replacement of host strand by donor DNA

Degraded host strand d)

Figure 11.25 Transformation in Bacillus subtilis. (a) A fragment of donor DNA become bound to the recipient cell surface by means of a DNA-binding protein. (b) After binding, a nuclease contained at the cell surface degrades one strand of the donor DNA, leaving the other strand to be ferried, by other transformation-specific proteins, to the interior of the cell. (c) A fragment of single-stranded DNA aligns with a homologous stretch on the recipient chromosome. (d) The donor fragment becomes integrated by a process of non-reciprocal recombination. At the next cell division, one daughter cell is a transformant, whilst the other retains the parental genotype

Degraded host strand d)

Figure 11.25 Transformation in Bacillus subtilis. (a) A fragment of donor DNA become bound to the recipient cell surface by means of a DNA-binding protein. (b) After binding, a nuclease contained at the cell surface degrades one strand of the donor DNA, leaving the other strand to be ferried, by other transformation-specific proteins, to the interior of the cell. (c) A fragment of single-stranded DNA aligns with a homologous stretch on the recipient chromosome. (d) The donor fragment becomes integrated by a process of non-reciprocal recombination. At the next cell division, one daughter cell is a transformant, whilst the other retains the parental genotype the recombinant genotype. In the latter, there is a stable alteration of the cell's genetic composition, and a new phenotype is expressed in subsequent generations.

Uptake of donor DNA from an unrelated species will not result in transformation; this is due to a failure to locate a homologous sequence and integrate into the host's chromosome, rather than an inability to gain entry to the cell.

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