Discussion:
Gene Regulation and Expression
1. Diphtheria is caused by strains of Corynebacterium diphtheriae that are lysogenic for a bacteriophage called "b". This phage carries a gene for an endotoxin (tox) that inhibits eukaryotic protein synthesis, and is a major virulence factor for this pathogen. Gene order in the b prophage is shown below:
2. Diphtheriae that are lysogenic for phage b produce endotoxin only under conditions where iron has become limited; excess iron inhibits tox gene expression.
3. Different classes of mutations in the coding sequence of a bacterial chromosomal gene called dtxR, which is not physically linked to phage b, lead to dramatic effects on Tox synthesis:
Class I: Mutations leading to the total loss of DtxR protein, which result in constitutive expression (expression all of the time) of phage b toxin regardless of how much iron is present.
Class II: Certain point mutations altering particular amino acids in DtxR result in the total loss of toxin gene expression, even when the cells are starved for iron.
A. Taking into consideration the 2 classes of mutations described above, indicate the role that DtxR plays in toxin gene regulation and the role of iron.
B. Based on the observations for Class II mutants, what is the relationship between iron and DtxR? Why do the Class II mutants shut off toxin expression completely?