Co-Transformation of Bacterial genome

Co -Transformation:

It is obvious that if the exogenous DNA that enters a recipient bacterial cell, contains known marker genes, say x, y and z, then these genes appear in the trans-formants, provided the segment or segments containing these genes are successfully integrated into the host chromosome. When x and y or x and z or y and z appear in the same trans-formant, the phenomenon is called co-transformation and the particular trans-formant is called a co-trans-formant.

Naturally, the probability of co-transformation and hence the frequency of co-trans-formants depend on the relative distance between the pair of marker genes. For detecting co-transformation, the recipient bacterium must have the corresponding recessive genes, x ,y– and z–, because only then the presence of the x, y and z genes can be detected. For convenience, the x, y and z genes may be represented as x+, y+and z+.

Co-transformation frequency may be used for preparing gene-maps. Thus, if it is observed that x+ y+ trans-formants appear more frequently than x+z+, it can be concluded that x+ and y+ are closer to each oilier than x+ and z+.

As transforming DNA generally consists of fragments, a particular fragment may or may not contain a marker gene. If the fragment taken up by a cell does not contain any marker gene, there will be no transformation of the marker genes, although other genes not taken into consideration may be present. If the fragment taken up by the recipient contains a marker x+ or y+, or both markers x+ and y+, the trans-formants may have the genotypes, x+x-, y+y- or x+x-y+y–. Only the last genotype represents a co-trans-formant.

Now, if the probability of x+x– and y+y– trans-formants in the population is 10-3each, then the probability of co-transformation of x+x–y+y– will also be 10-3provided x+ and y+ are present in the same DNA fragment. But if x+ and y+ are present on different DNA fragments, the probability of taking up the two fragments simultaneously will be 10-3 x 10-3 i.e. 10-6.

The same argument holds good for all the pairs. The probability of x+, y+ and z+occurring on different fragments being co-transformed is much less, in the order of 10-3 x 10-3 x 10-3 i.e. 10-9. On the other hand, if x+, y+ and z+ occur in the same fragment, the probability of the three genes being co-transformed will be 10-3. Thus, from the probability measurements, it is possible to construct a gene map showing relative distances between the genes, as well as their order in the chromosome.

The principle of using co-transformation as a tool for gene mapping is illustrated in Fig. 9.99: