Question

Need help and a explanation with this please, don’t understand how to approach this question nor what B and K cells have to do with this problem

Answer 1

The two E.coli strains K (l) and B are hosts for rII mutants of T4 phage (T4 rII). These T4 rII mutants produce large ragged plaques due to mutation in rII (r= rapid lysis) loci in T4 phage. These mutants are conditional mutants that grow only on particular set of condition

E. coli B allows wild type as well as rII mutant to grow. However, wild type phages (r+) produce small plaques (1-1.5 mm diameter) while rII produce large plaques (2-2.5 mm diameter). Hence, it is permissive for the rII mutants. E. coli K (lambda) does not allow rII mutants to grow. This strain is lysogenic for lambda phage. However, wild-type phages to can grow in E.coli K strain. The K strain is non permissive for rII mutants.

There are two types of rII mutants, rIIA and rII B. The polypepide of A and B regions of rII have distinct functions. A polypeptide has an unknown function while B polypeptide is a cell membrane protein.

When E. coli K is infected simultaneously with two mutants, if mutation is in different genes, one functional copy of each gene gets expressed between them. As a result, the phages will replicate and lyse the K cells. This is complementation. If mutation is in same gene, then no functional copy is present in E.coli. Hence, there is no replication and cell lysis.

In the experiment, the rIIA and rIIB mutants together are infected at high multiplicity (more phages per bacterial cell). Only if complementation occurs between the two mutants, will a wild type progeny be formed, which lyses the K cells. This progeny is plated at low multiplicity (less phages per bacterial cells). If the progeny phage is a wild type phage, it will infect the K cells and give rise to plaques. Hence, plaques will be seen. The phages will also grow on B strain giving rise to small plaques.

If not, no progeny phages will be seen as the phages cannot replicate independently in K strain. Hence, when they are plated on K strain, no plaques will be seen. On B strain, large mutant rIIA and rII B colonies will be seen which are from the original mixture.

Right option is a. complementation.