Question

Please provide a thorough response with an explanation for each, this is a study question to prepare for a test. I would like to understand the reasons provided for each answer :)

4. What would be the likely phenotype of a lambda phage (lytic vs. lysogenic) carrying the following mutations? Explain the logic of why you think (some of these may require to think about them-and they may have more than one possible theoretically justifiable answer) each answer is correct. UBe diagram3 a3 mece3sar a. c b. cIl d. cro- e. PRM f. PRE g. ORl- (next to Pa) h. oR3- (next to PM)

Answer 1

Let me show you a rough picture first what:

So now,

a)CI - : CI when present maintains the lysogeny state of the virus. Even if the virus gets integrated with the help of integrase, it will excise out immediately due to absence of CI. So the virus gets to have only lytic phase.

b) CII- : CII is the one that activates P(RE) promoter which produce the CI or repressor protein. So if no CII, no CI is produced, and as said earlier, if no CI, no lysogeny . So only lytic state again.

c) CIII- : CIII protein has 2 functions . It stabilizes CII protein, and protects it from host protease FtsH so that it doesn't gets deactivated . So lack of CIII means, no CII which men's no CI, which means only lytic state .

d) Cro- : Cro protein from Cro gene is the one which is responsible for the lytic state. It represses CI gene activation, and also makes sure that other lytic genes are encoded. So lack of cro gene means, no lytic state .so the virus can do only lysogeny.

e) P(RM)- : P(RM) promoter is also called as promoter for repressor maintenance,i.e., it maintains the amount of CI in the cell .firstly the CI is encoded from P(RE) promoter but later it is encoded from P(RM) promoter . So lack of this promoter means, though initially the viral genome gets integrated due to presence of CI,but as it is not maitined, no CI, which means the genome gets excised out and then has only one option : lytic cycle.

f) P(RE)- : this promoter stands for promoter for repressor establishment i.e, it establishes the CI amount in bacterial cell . cI is first produced with the help of this promoter . So if P(RE) is absent , no CI is produced. No lysogeny .onlyonly l cycle.

G) OR1- : as you can see in the picture, OR1 lies near cro gene, exactly near P(R) promoter. When CI starts accumulating in cell, it first binds to OR1 to make sure no cro protein is produced which can make it problematic for CI to get into and maintain a lysogeny state .So it needs to shut down Cro protein right? cI does it with the help of OR1. But if OR1 is absent , no binding of CI to OR1, Cro protein is not repressed.

h) OR3- : Cro protein when abundant in cell binds to OR3 to stop CI from being produced just like you read in the above explanation. So if no OR3, Cro can't stop CI, and if CI starts accumulating? There is a chance that the virus may enter the lysogenic state.

Hope it helped !