Gene Complementation in Bacteria

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Lecture 14
Lecture
Gene Complementation in Bacteria
In order to perform tests for dominance or for complementation in bacteria we need a way to make the bacteria diploid for part of the chromosome. To do this we need to consider a different extrachromosomal element:

Ori T
The F plasmid
(length 105 base pairs)
Tra
genes
There are some special terms to describe the state of F in a cell: F– refers to a strain without any form of F, whereas F+ refers to a strain with an F plasmid. F,

Donor cell

Recipient cell
F pilus
Ori T

F is very efficient at transferring itself from an F+ cell to an F– cell. After culturing F+ and F– cells together about 1/10 of the F– cells will become F+. The property that makes F useful for genetic manipulation is that at low frequency the plasmid will integrate into chromosome. This occurs because F carries insertion sequences that are also present at multiple locations on the chromosome. Crossing over between insertion sequences on F and on the chromosome gives integration.

Hfr: a strain with F integrated into the chromosome that will give efficient transfer of Hfr
some chromosomal markers.
F+ plasmid: 1) Transfers itself at a frequency of 0.1
2) Does not transfer chromosomal markers
Hfr
Hfr

1) Transfers some chromosomal markers efficiently
2) Other markers transferred inefficiently - Gradient of transfer (It takes about 100 minutes to transfer the entire chromosome)

Consider an F+ integrating to make an Hfr:
Hfr

A

B

A

C

D

B

C

D

This process can be reversed to go back to the F+ state:

F+
A

B

C

D

The recombination can occur at a different position to give an F plasmid that carries a part of the chromosome. This form of F is called an F’’. F

B

C
A

B

D

C

F’’s are usually isolated by selection for early transfer of a marker that is transferred F
late in the Hfr. In the example above the F’’ could have been isolated from a population Hfr
F...
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