When we have a positive regulation the control is tight, so we have low background expression under non-induced conditions.
Negative regulation can not be fully controlled, but as far as I understand it is more popular than positive regulation.
Why?
Are there any other important features of these two types of regulations?
Thank you in advance!
usually inducible promoters are used to drive the expression in late exponential and stationary phase... products may be toxic, induce cell lysis or slowing their growth...you need grow larger biomass and then start the production of proteins....
Please go through the attached document. I am sure you will find what you seek
I think many of the respondents are mixing up the difference between positive and negative regulation, which is defined as the behavior of the regulatory protein, which is not the same as a inducible or co-repressible system. For example in E. coli both the arabinose operon and the lactose operon are inducible by the sugar, but lac operon is primarily controlled by a repressor and hence negative regulation whereas ara operon is positively regulated by a transcriptional activator (I realize that is an oversimplification for both but primarily correct).
For controlling gene expression an inducible system is normally easier to regulate because you just need to add the small molecule inducer to turn on gene expression, whereas in a co-repressible system you would need to remove the co-repressor (frequently an amino acid or something similar). So from that context inducible is much easier.
Whereas the difference between a negatively regulated vs a positively regulated system is about the behavior of the regulatory protein. The problem with a negatively regulated system is that for controlling a multi copy plasmid you need to have excess repressor present, and usually a single copy of the repressor gene on the chromosome does not suffice. So often negatively regulated promoters are not as tightly regulated and may have substantial basal expression. Whereas for positive regulation you don't have the same difficulty and the basal uninduced expression levels are usually lower (more tightly regulated).
As to why negative regulation is more common in plasmid systems, I think it primarily is historical. The lac operon was extremely well studied and understood in E. coli at the time many cloning and expression plasmids were being developed and it has just stayed with us.
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