How Do Plasmids Influence Bacterial Gene Expression and Antibiotic Resistance?

[gemma786]

Hi
I want to know about the role which plasmids play in a baterial cell.How the genes which are coded in plasmid DNA expressed themselves ? Does the transcription of plasmid DNA takes place independently of chromosomal DNA I mean to say that do the plasmids have promotor sequence for RNA polymerase to bind or plasmid first bind to chromosomal DNA forming episome inorder to express the genes which it contanins ? AND how the presence of antibiotics in the environment influence the number of plamids in a bacterial cell ?
And what is the effect of having large number of plasmid of same type in a bacterial cell ?
Please help me to find answer to these questions.
Thanks.

 

[iansmith]

How the genes which are coded in plasmid DNA expressed themselves ?
Does the transcription of plasmid DNA takes place independently of chromosomal DNA I mean to say that do the plasmids have promotor sequence for RNA polymerase to bind or plasmid first bind to chromosomal DNA forming episome inorder to express the genes which it contanins ?

Basically, expression of plasmid gene is mostly independent but it is partly dependent on the bacterial host expression apparatus. The plasmid genes have their own promoter but, in most cases, they need the bacterial host RNA polymerase for their transcription. Some plasmids have their own regulators.

AND how the presence of antibiotics in the environment influence the number of plamids in a bacterial cell ?

It depends if the plasmids codes for an antibiotic resistance genes and if a large number of the gene product encoded by the resistance genes is needed for resistance to be achieved. It will also dependent on the type of origin of replication of the plasmid. The presence of antibiotics may have no effect on the number of plasmids but in some cases it may results in increase number of plasmid copy number if the plasmids has an antibiotic resistance gene.

And what is the effect of having large number of plasmid of same type in a bacterial cell ?

Again it depends if the plasmid is fairly large or small but a large number of plasmid will have a metabolic cost due to its large number of replication events.

 

[gemma786]

Thanks. jansmith and sorry for replying late.
I was just trying to understand how plasmid can be used as a cloning vector.
And as you said plasmids containing a gene for antibiotic resistance can multiply their copies if surrounding medium contains antibiotic but you also said that plasmids can also have regulators now does this means to say that plasmids have operon units like dna ? It would mean that in order clonise a plasmid, we would have to take into consederation those operator units and the products which would lead to activation of those operon units , because we want to make as many copies of plasmid as we can in cloning process along with the desired gene that we have introduced in plasmid , isn't it ?
I am not sure about this . I need help !

 

[iansmith]

In nature, plasmids that are very small will only have one gene, which might be required for their replication, but very large plasmid can contain several genes, including genes that are required for their replication and partitioning into the daughter cells.

For cloning vectors, most plasmids were derived from naturally occurring plasmids but the cloning vector went through major modification and removal of DNA with coding regions. In some case, these modification had major impacts.

Copy number of plasmids are often controlled through the origin of replication. For example, pUC19 or pUC18 are high copy numbers and their source plasmid is pMB1 is a low copy number plasmid. pUC19/18 have a point mutation in their origin of replication which allows for the high copy number but they also lack a rop gene.