Conjugation is by no means the only way in which bacteria can acquire foreign DNA. As early as 1928, Frederick Griffith had demonstrated the phenomenon of transformation of bacterial strains by the transfer of some factor. However, he did not know the nature of that substance. It was not until 1944 that Oswald Avery, Colin MacLeod, and Maclyn McCarty showed that genetic transformation was caused by the trans port of DNA, through solution, into the transformed bacterium. These experiments are at the very basis of molecular biology.
Finally, DNA can be transferred between bacteria via bacteriophage, by a process called transduction (Figure 1). This usually involves temperate phage, which can exhibit two alternative life cycles: lytic and lysogenic. When in the lytic cycle, the virus can enter the bacterial cell, replicate, lyse the host bacterium, and go on to infect other bacteria. In the alternative lysogenic cycle, it can integrate its DNA into the bacterial chromosome. There it may remain for many bacterial generations in a dormant state, until some stimulus, such as radiation or chemical insult, causes it to be released from the host genome. It will then form new viruses and kill the bacterium. Packaging of the viral DNA into viral particles is a low-fidelity process: small pieces of bacterial DNA may become packed, alongside the phage genome, and thus are transferred to the newly infected cell. At the same time, phage genes can be left behind in the bacterial chromosome into which they had been integrated. The phage can be modified by addition of foreign DNA, and can act as a vector to insert this into bacteria. We discuss the role played by transformation and transduction in genetic engineering in Chapter 5.
Fig1. Conceptual schematic of transduction. Steps involved in transduction are as follows: (Step 1) A phage infects a bacterial cell. (Step 2) Phage DNA enters cell and replicates; phage proteins are made; bacterial chromosome is broken down. (Step 3) Occasionally, pieces of bacterial DNA are packaged into the phage head; some viral particles released during cell lysis contain bacterial DNA. (Step 4) A phage carrying bacterial DNA infects a new bacterial cell. (Step 5) Recombination between the donor bacterial DNA and the recipient bacterial DNA can occur; the recombinant cell is different from both the donor and the recipient cell.
