Horizontal Gene Transfer (HGT) PDF

Horizontal Gene Transfer (HGT) Horizontal gene transfer (HGT) also known as lateral gene transfer a process in which the genetic material is transferred from one organism to another organism that is not its direct descendant ‘the non-genealogical transmission of genetic material from o...

Horizontal Gene Transfer (HGT) Horizontal gene transfer (HGT) also known as lateral gene transfer a process in which the genetic material is transferred from one organism to another organism that is not its direct descendant ‘the non-genealogical transmission of genetic material from one organism to another’ a mechanism that permits the acquisition of evolutionary novelties Three mechanisms: Horizontal gene transfer occurs via three mechanisms: 1. Transformation 2. Transduction 3. Conjugation TRANSFORMATION the uptake of exogenous DNA by a competent cell involves recombination and integration of the foreign DNA into the recipient’s genome Sources of exogenous DNA can be: (1) native bacterial chromosome fragments, (2) plasmid, or (3) bacteriophage DNA. transformation does not require cell-cell contact. RecA proteins promotes genetic exchange between a fragment of the donor's DNA and the recipient's DNA Competence The ability to take up DNA from the medium or environment Can be natural or artificial Natural Competence usually occurs as a consequence of nutritional shift-down and coincides with a decrease in rate or a blockage of DNA synthesis Examples: Bacillus subtilis, Streptococcus pneumoniae, Haemophilus influenzae, Neisseria gonorrhoeae, Helicobacter pylori, Acinetobacter baylyi, and some marine cyanobacteria Artificial Competence and Transformation Bacterial cells that are not naturally transformable can be induced to take up foreign DNA via artificial competence. Artificial competence is not coded by the bacterial genes. It is a procedure performed in the laboratory. Achieved through (1) chemical treatment and heat shock, or (2) electroporation Transformation through chemical treatment and heat shock method Transformation through electroporation CONJUGATION a form of horizontal gene transfer in both gram- negative and gram-positive bacteria that requires cell-to-cell contact (mating) a plasmid-encoded mechanism requires a donor cell (contains the conjugative/F plasmid) and a recipient cell (does not have a conjugative/F plasmid)  General Mechanism of Bacterial Conjugation 1. Pilus Formation- The donor cells (F+ cells) form a sex pilus and begin contact with an F- recipient cell. 2. Physical Contact between Donor and Recipient Cell- The pilus forms a conjugation tube and enables direct contact between the donor and the recipient cells. 3. Transfer of F-Plasmid- The F-factor opens at the origin of replication. One strand is cut at the origin of replication, and the 5’ end enters the recipient cell. 4. Synthesis of Complementary Strand- The donor and the recipient strand both contain a single strand of the F-plasmid. Thus, a complementary strand is synthesized in both the recipient and the donor. The recipient cell now contains a copy of F plasmid and becomes a donor cell.  The transfer of the F plasmid converts an F- recipient cell into an F+ cell rolling circle replication - a mechanism adopted by certain plasmids, among other genetic elements represents one of the simplest initiation strategies, that is, the nicking by a replication initiator protein on one parental strand to generate the primer for leading-strand initiation and a single priming site for lagging-strand synthesis Transfer of plasmid DNA by conjugation. (left photo) Transfer of the F plasmid converts an F- recipient cell into an F+ cell. Note the mechanism of rolling circle replication in steps 4 and 5. (b) Details of the replication and transfer process. Note the large number of proteins needed for successful DNA transfer. Hfr cells (high frequency of recombination) Refers to the high rates of genetic recombination between genes on the donor (Hfr) and recipient (F-) chromosomes. Both F+ and Hfr cells are donors, but unlike conjugation between an F+ and an F-, conjugation between an Hfr donor and an F- leads to transfer of genes from the host chromosome. Transfer of Chromosomal Genes by Hfr  TRANSDUCTION a bacterial virus (bacteriophage) transfers DNA from one cell to another 1. generalized transduction, DNA derived from any portion of the host genome is packaged inside the mature virion in place of the virus genome. the bacteriophages can pick up any portion of the host's genome 2. specialized transduction, DNA from a specific region of the host chromosome is integrated directly into the virus genome—usually replacing some of the virus genes. ex: phage lambda the bacteriophages pick up only specific portions of the host's DNA Bacteriophages viruses that infect bacteria may have a lytic cycle or a lysogenic cycle, and a few viruses are capable of carrying out both infection by a bacteriophage results in the production of new virions Lytic Cycle lytic phages bacterial cells are broken open (lysed) and destroyed after immediate replication of the virion ex: T4 lytic bacteriophage  Lysogenic Cycle does not result in immediate lysing of the host cell “temperate phages” viral genome will integrate to the host DNA and replicate along with it, or may even become established as a plasmid virus remains dormant until host conditions deteriorate, then the endogenous phages (known as prophages) become active = initiate the reproductive cycle leading to lysis of the host cell  Lysogenic Cycle lysogenic cycle allows the host cell to continue to survive and reproduce, the virus is reproduced in all of the cell’s offspring. example of a bacteriophage known to follow the lysogenic cycle and the lytic cycle: phage lambda of E. coli phage DNA is incorporated into the host genome, where it is passed on to subsequent generations. Environmental stressors such as starvation or exposure to toxic chemicals may cause the prophage to excise and enter the lytic cycle. Generalized Transduction the bacteriophages can pick up any portion of the host's genome  Specialized Transduction the bacteriophages pick up only specific portions of the host's DNA