If the F factor integrates at random points in the chromosome Hfr (high frequency of recombination) strains are generated.
Episome (附加体) : a plasmid that can integrate into the chromosome
Mating types in bacteria
Bacterial Cells Can Recombine Discovered by Lederberg and Tatum (‘40s)
野生的
Bacterial Cells Can Recombine
Why did they use multiple different mutations?
F plasmid
(2) Dissimilation plasmids
Encode enzymes for catabolism of unusual compounds
(3) R factor
Encode resistance
antibiotic
Conjugative plasmid
有性纤毛基因: Pilus
Multiple Integration Sites
One can map the whole E. coli genome by using different Hfr strains.
Conjugation
Definition: Gene transfer from a donor to a recipient by direct physical contact between cells
This can be used to map genes in E. coli.
Note: F factor is the lastest
Multiple Integration Sites
The F plasmid can integrate into the E. coli genome at a number of different places. Thus, one can map the whole E. coli genome by using different Hfr strains.
(1) Extrachromosomal DNA, usually circular
(2) Usually encode ancillary functions for in vitro growth.
(3) Can be essential for specific environments: virulence, antibiotics resistance, use of unusual nutrients, production of bacteriocins (细菌素).
The process is called congugation (接合), and it involves the transfer of a plasmid (F factor) from one cell to another
F Factor Integration Yields Hfr Strains
Conjugation (接合): the second mechanism that bacteria use to exchange genes
E. Coli - the ultimate lab rat
Plasmid (质粒) : Extrachromosomal genetic elements that are capable of autonomous replication (replicon)
Figure 8.29
F plasmid
(1) large (100 kb) (2) low copy (1-2 copies/cell) (3) self transmissible (传递) (4) requires protein synthesis
RepE gene encodes RepE protein
(4) Must be a replicon - self-replicating genetic unit.
Classification of Plasmids
(1) Conjugative plasmid
Carries genes for sex pili and transfer of the plasmid,
Transferred DNA May Recombine
But the transfer is unidirectional, unlike eukaryotic sex.
外基因子 内基因子
Partial diploid (部分二倍 体) or meorzygote (部分 合子) p 227
重组子
Recombination Requires Physical Contact
This shows that it is not the transfer of a molecule through the medium
过滤器
Recombination Requires Physical Contact
接合后体
The Order of Transfer Reflects Gene Order
This can be used to map genes in E. co
乳糖 半乳糖
The Order of Transfer Reflects Gene Order
RepE protein binds to origin of replication and initiates DNA replication. RepE binds to the repE promoter and activates transcription.
Bacterial Phenotypes