Question

T. Miyake and M. Demerec examined proline-requiring mutations in the bacterium Salmonella typhimurium (T. Miyake and M. Demerec. 1960. Genetics 45:755–762). On the basis of complementation testing, they found four proline auxotrophs: proA, proB, proC, and proD. To determine whether proA, proB, proC, and proD loci were located close together on the bacterial chromosome, they conducted a transduction experiment. Bacterial strains that were proC⁺ and had mutations at proA, proB, or proD were used as donors. The donors were infected with bacteriophages, and progeny phages were allowed to infect recipient bacteria with genotype proC^?proA⁺proB⁺proD⁺. The recipient bacteria were then plated on a selective medium that allowed only proC⁺ bacteria to grow. After this, the proC⁺ transductants were plated on selective media to reveal their genotypes at the other three pro loci. The following results were obtained:

Donor genotype	Transductant genotype	Number
proC+proA?proB+proD+	proC+proA+proB+proD+	2765
	proC+proA?proB+proD+	3
proC+proA+proB?proD+	proC+proA+proB+proD+	1838
	proC+proA+proB?proD+	2
proC+proA+proB+proD?	proC+proA+proB+proD+	1166
	proC+proA+proB+proD?	0

a) Why are there no proC^? genotypes among the transductants?

b)Which genotypes represent single transductants and which represent cotransductants?

c)Is there evidence that proA, proB, and proD are located close to proC? Explain your answer.

Answer 1

a) Among the trunsductants there are no pro C- genotype because the screening was only performed in the presence of proC-

b) proC+proA+proB+proD+ are the genotype of wild type and single transductants are represented by those genotype on the other hand proC+ proA+ proB+ proD+ and proC+ proA+ proB- proD+ are the cotransductants of proC+ proA- and proC+ proB-

c) From the above data it is clear that the proA and pro B are located close to the proC but the pro D is not.