Bacterial Genetics and Bacteriophages

Bacterial Genetics and Bacteriophages

Bacteriophages

Abacteriophageis a virus that infects bacteria. Like all viruses, phages are obligate intracellular para-sites, devoid of protein synthesizing machinery and energy conversion systems. They contain nucleic acid enclosed in a protein coat, or capsid. Bacterio-phages require a living bacterial host in which to carry out their reproductive cycle. 

Thelyticorvegetative life cycleculminates in lysis of the host cell and the release of numerous viral progeny. Bacterial viruses exhibiting only a lytic life cycle are known as virulentbacteriophages because they eventually cause the death and destruction of the host bacterium. For ex-ample, the T-even phages such as T2, T4, and T6 are virulent.

The life cycle of T4 is illustrated in Figure 7-1. The lytic cycle con-sists of five steps. The cycle begins by attachmentof the bacteriophage to molecules on the host’s cell wall. Next, the virus introduces its genet-ic material into the cell (penetration). Once the bacteriophage genome enters the cytoplasm, it subverts the host’s nucleic acid and protein syn-thesis apparatus and initiates the synthesisof viral proteins and DNA. As the viral proteins are synthesized, they self-assemble into viral compo-nents such as the head (containing the phage DNA), tail, and tail fibers. Theassemblyprocess results in the formation of numerous intact phage particles within the cell. After assembly, viral proteins cause lysis of the host cell, and all the viral progeny arereleased into the environment.

A temperatephage can cause a lytic infection, but can also exist within the host bacterium as a prophage, that is when the genetic mate-rial of the phage is inserted into the DNA of the host cell. In this state, the lysogenizedbacteria can carry out a seemingly normal life cycle. When exposed to UV light or nutrient deprivation, the prophage is excised from the bacterial genome, leading to a lytic cycle.

Recombination

Geneticrecombinationin bacteria is a nonreciprocal process whereby segments of genetic material from two different sources are brought to-gether into a single DNA molecule. Homologous recombination was dis-cussed in Chapter 3. Site-specific recombinationinvolves the recombi-nation of two DNA molecules at specific locations variously called insertion sequences (IS), long terminal repeats (LTRs), and attach-ment sites(att). The integration of the bacteriophage λinto the E. coli chromosome is a common example of site-specific recombination in-volvingatt(see Figure 7-2). Both possess attsites, which are recognized by the λintegration and excision enzymes. Both chromosomes share a short region of homology indicated by “O.” This region of homology is flanked by short DNA sequences that are unique to the organism. The flankingE. coliregions are indicated by B and B , while those of the λ phage are indicated by P and P . After integration of the phage DNA into that of E. coliby site specific recombination, the λchromosome is flanked by the sequences BOP and POB .

Genetic Transfer

Transformationis a mechanism of genetic transfer between bacteria in which the donor DNA exists cell-free in the recipient bacterium’s immediate environment. DNA can be naturally released into the environment when cells die and subsequently lyse. Experimentally, DNA containing genes of interest, usually within a plasmid, can be introduced into the en-vironment in order to transform bacterial cells. The ability of a recipient bacterium to take up free DNA and become transformed is known as competence. Some strains of bacteria are naturally competent. In others, competence is a brief physiological state during the exponential growth phase; in these bacteria, Ca²⁺ ions enhance the level of competence.

Transduction is a mechanism of DNA uptake by bacteria in which the donor DNA, consisting of fragments of the bacterial chromosome, is introduced into a bacterial cell via a phage vector. In generalized trans-duction, virtually any bacterial gene can be transferred by a lytic bacte-riophage. During packaging of the viral DNA into the capsids, some of the host’s DNA may be packed into the virus along with an incomplete

viral genome. This virus will be able to initiate infection, and therefore introduce the original host’s DNA into a new bacterial cell, but will not be able to replicate itself or lyse the new host cell.

Specialized transductionis a process whereby a lysogenic bacte-riophage serves to transfer a specific gene at a high frequency. When lysogenic bacteriophages infect host cells, their DNA is incorporated into the host’s genome by site-specific recombination, which always occurs at a specified location and adjacent to certain genes. Through the process ofinduction, the prophage genome becomes excised from the host and undergoes a lytic cycle. Occasionally, the phage excision from the host’s genome is defective and results in the release of a viral genome that con-tains part of the host’s genome, in particular, those genes that are adja-cent to the phage’s site of insertion.

Conjugationis a process during which genetic information is trans-ferred unidirectionally from a donor bacterium to a recipient through a cytoplasmic channel between the two cells. The required cell-to-cell con-tact between the donor and recipient can be achieved through sex pili, through agglutinins(substances that promote cell clumping), or via pheromones(chemicals that alter the behavior of other members of the

same species).