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F+, F-, Hfr and F’ Cells – What is the Difference?

F+, F-, Hfr and F’ Cells – What is the Difference?

We talk about F+, F–, Hfr and F’ Cells when we talk about bacterial conjugation which is one of the modes of horizontal gene transfer in bacteria. And it is important that we understand what these cell types are and what is the difference.

  • F+ Cells:

These are the bacterial cells which contains the F plasmid. They are designated F+ simply because they have F plasmid. We know plasmid is an extrachromosomal DNA that can replicate independently. It is called F plasmid because it has F factor which is Fertility factor. This fertility factor contains the genes require for the transfer or conjugation.

F+ Cells = Cells containing F plasmid (F plasmid = Plasmid containing F factor) 

Figure 1. F+ Cells

  •  F– Cells:

F– cells are the cells without F plasmid. These cells act as recipient cells because they don’t have F plasmid and thus they cannot donate the genetic material. They are designated as F– simply because they do not have F plasmid.

F– Cells = Cell lacking F plasmid

Figure 2.  F– Cells

  • Hfr Cells: High frequency of recombination

We just talked about F plasmid. This F plasmid is an episome which means it can either exist as an independent unit or it can integrate itself into chromosomal DNA. This integration is possible because F plasmid also contains the insertion sequence and via homologous recombination it can integrate itself. So when this integration happens, the resultant cells or the derivative of F+ cells is called Hfr cells.

Why are they called Hfr? – When these Hfr cells cross with F– cells; along with F plasmid some part of donor chromosomal DNA also gets transferred in the recipient cells because F plasmid is integrated into donor chromosomal DNA. Therefore the recipient cells now have got three different types of DNA – its own chromosomal DNA, some part of F plasmid along with some part of donor’s chromosomal DNA. So because crossing with Hfr cells results in high degree of recombination in recipient cells such cells are called Hfr cells.

Hfr Cells = Derivatives of F+ cells where F plasmid has integrated into host chromosomal DNA

Figure 3. Hfr Cells

  • F’ Cells:

We just discussed that F plasmid is an episome so once it gets integrated into host chromosomal DNA, it can also disintegration itself back from the host chromosome. Sometime this disintegration is not accurate and while disintegrating from host chromosome; F plasmid might pick up some genes next to its disintegration sites. So such cells where the F plasmid has disintegrated and have got some genes of host chromosome are called F’ cells. They are the derivatives of Hfr cells.

F’ Cells = Derivatives of Hfr cells where F plasmid has disintegrated from host chromosome and picks up some host genes next to F plasmid integration sites

Figure 4. F’ Cells

Figure 5. Summary 

I hope this post helps 🙂

To understand more details, watch a video on this topic here.

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Agrobacterium – Nature’s own genetic engineer!!

Agrobacterium – Nature’s own genetic engineer!!

Agrobacterium is nature’s beautiful creation and it is nature’s own genetic engineer. This microscopic genius has the ability to transfer its gene to plant and this property has made Agrobacterium very popular in the field of genetic engineering. Because utilizing this property scientists are able to obtain improved quality plants.

Agrobacterium is Gram negative soil bacteria and when it infects plant i.e. when it transfers its gene to plant, the plant develops Crown Gall disease. So in this post we will see the important genes involved in the transfer and of course at the end I will share my trick to remember which gene does what function. So let’s get started 🙂

Agrobacterium’s gene transfer property is within its plasmid called Ti plasmid. Ti stands for Tumor Inducing.

Figure a shows the important segments of Ti plasmid which are as followed.

  1. Ti plasmid has T-DNA region which is transfer region and it is the only part which gets transfer from bacteria to plant. This T-region contains auxin production, cytokinin production and opine synthesis genes.
  2. Ti plasmid also has virulence region which is require to mediate the gene transfer. This region includes genes such as vir A, vir B, vir G, vir C, vir D and vir E.

Mechanism of Gene Transfer :

  •   When a plant gets injured (may be by insect bite), it causes secretion of phenolics such as acetosyringone from plant. This acetosyringone acts as attractant for Agrobacterium. As a result, lot of Agrobacterium would get attracted towards the wounded region of the plant.
  •   Acetosyringone activates the vir A which is a transmembrane protein by autophosphorylating it. Once vir A is activated it activates vir G by phosphorylating vir G. Now protein G is transcription factor which means once it is activated it transcribes remaining genes of vir region i.e. vir B, vir C, vir D and vir E.
  •   The first vir gene to be transcribed is vir C, which makes a complex with right T-DNA border.
  •   Second gene in the line is vir D which act as endonuclease. It recognizes the right T-DNA border + protein C complex and it will digest or form a nick in that region.
  •   Third event is formation multiple copies of vir E gene. Now this multiple copies of protein E would bind to the nicked portion of T-DNA and escorts it from bacteria to plant.
  •   But wait, how would it take T-DNA from bacteria to plant? Is there any channel or bridge between both? This problem is solved by vir B gene which provides bridge between the bacteria and plant and T-DNA is transferred through this.
  •   Following the transfer of T-DNA to plant, it gets inserted into the chromosome of plant. Now remember we just saw that T-DNA has auxin production, cytokinin production and opine synthesis genes? So these genes will now start getting expressed. When auxin and cytokinin genes are expressed, it causes uncontrolled growth of plant cell resulting into tumor or Crown Gall disease.
  •   What about opine synthesis gene? When it is expressed, it secretes opine into the soil which attracts the soil Agrobacterium and they will utilize this opine as their nutrient source.

 

Wonderful mechanism, isn’t it? Now my favorite part!! How to remember functions of these genes? What I do is, I correlated their function with what a gene is called. Like,

A = Autophophorylation

G = Gene transcription

C = Complex formation with T-DNA

D = Digestion

E = Escorting T-DNA

B = Bridge formation

 
For more explanation watch this video.