Question

In a general transduction experiment, P1 phage are grown on the trp+ lac+ ampR tetS ecoli abd are ysed to transduce E. coli that are trp-, lac- ampS tetR. What would you supplement inimal media (agar, salts) with to identify the trp+ lac+ ampR tetR transductants that you plate on these media?

Answer 1

Here in the question we have been told that we are to test for trp+ lac+ ampR tetR transductants after general transduction experiment via P1 phage.

Before we select for test for transductants with all 4 tranduced genes/operons, we need to understand the meaning of trp, lac, ampR/S, tetR/S

i.) trp operon : Refers to trp operon that is responsible in regulation of expression of genes for synthesis of tryptophan. This operon funtions as repressible system where tryptophan functions as repressor. In trp+ case, we have a functional trp operon where in presence of tryptophan in media, the genes are repressed and tryptophan in media is utilised. While in the case when tryptophan is not present in media for trp+ operon transductant, the transductant has no repressor (tryptophan) in media and thus genes are expresseed for tryptophan synthesis for its growth/requirement. trp- transductant lacks the whole system and can survive only in presence of tryptophan in media and will die if no tryptophan is provided in media.

ii) lac operon: refers to lac operon which is a inducible system. lactose/allolactose serves as the inducer. Inducer binds to the repressor molecule responsible for repression and thus inhibits repressor to bind and leads to induction of downstream genes. Bacteria utilise glucose as primary source of energy and also inhibits lactose from binding to repressor. Thus, when glucose is present in the media, operon is in state of repression. It does not matter if lactose is present or not, if glucose is present in the media, the operon will be in state of repression. Only when there is no glucose in media to be utilised and presence of loctose/allolactose in media, only then the downstream genes are induced and expressed.

iii)amp : refers to ampicillin & tet: refers to tetracycline antibiotics. ----- R: refers to resistant and S: refers to sensitive. ampR means presence of ampicillin resitance an similarly are the case for ampS tetR and tetS

Now that we are clear with basics we can answer the question to screen for trp+ lac+ ampR tetR transductants.

We need a media without any tryptophan such that we induce trp+. We need no glucose in media and need lactose as source of carbon/energy such that we induce lac+. As we need to screen for ampicillin and tetracycline resistant transductants, we will have ampicillin and tetracycline in the media.

So there you have it. If you are clear with how operons work and antibiotic sensitivity/resistance concept and that we have genes that grant resistance to a bacteria against an antibiotics, it should be really easy to ace in such question. Would appreciate your feedback.