Question

Flagellar phase variation occurs in Salmonella and involves several genes and DNA sites. Several different mutations were made in those genes/sites to determine the effects on phase variation. For each mutation listed below, which flagellin is synthesized when the invertible region is in the position indicated? Note: A null mutation results in the production of a nonfunctional product or no product at all. Assume base changes result in inactivation of that site so it can no longer function. A nonpolar mutation does not affect expression of downstream genes in an operon.

FljB FliC Both FljB and FliC Neither FljB nor FliC  

A. A null mutation in the hin gene with the invertible region in the ON position.
B. Mutations in the recombinase recognition sites in the recombination sites flanking the invertible region, with the invertible region in the OFF position.
C. A polar null mutation in fljB with the invertible region in the OFF position.
D. A null mutation in fljA with the invertible region in the OFF position.

Answer 1

Answer A:-

1. A null mutation in the 'hin' gene would basically affect the synthesis of the product namely Hin Recombinase which is responsible for inversion of the DNA segment involved in synthesis of flagellar protein variants that acts to protect the bacteria against immune response by changing the surface antigens.
2. When the invertible region is ON, it indicates that the Flagellin B will be synthesized however on the other hand, there will also be synthesis of Repressor protein that will inhibition synthesis of Flagellin C protein. This is the regulatory control when Hin Recombinase is not synthesized due to null mutation as it is not able to work out with Hix to bring about inversion.