Chapter 20, Problem 20.2PS

The Pickled Prokaryote. Pickelensia hypothetica is an imaginary prokaryote that converts a wide variety of carbon sources to ethanol when cultured anaerobically in the absence of ethanol. When ethanol is added to the culture medium, however, the organism obligingly shuts off its own production of ethanol and makes lactate instead. Several mutant strains of Pickelensia have been isolated that differ in their ability to synthesize ethanol. Class I mutants cannot synthesize ethanol at all. Mutations of this type map at two loci, A and B. Class II mutants, on the other hand, are constitutive for ethanol synthesis: they continue to produce ethanol whether it is present in the medium or not. Mutations of this type map at loci C and D. Strains of Pickelensia constructed to be diploid for the ethanol operon have the following genotypes and phenotypes:

$\begin{array}{l}{A}^{+}{B}^{-}{C}^{+}{D}^{+}/A^{-}{B}^{+}{C}^{+}{D}^{+}\text{inducible}\\ A^{+}{B}^{+}{C}^{+}{D}^{-}/A^{+}{B}^{+}{C}^{+}{D}^{+}\text{inducible}\\ A^{+}{B}^{+}{C}^{+}{D}^{+}/A^{+}{B}^{+}{C}^{-}{D}^{+}\text{constitutive}\end{array}$    (20-2)

(a) Identify each of the four genetic loci of the ethanol operon.

(b) Indicate the expected phenotype for each of the following partially diploid strains:

i. A⁻B⁺C⁺D⁺/A⁻B⁺C⁺D⁻

ii. A⁺B⁻C⁺D⁺/A⁻B⁺C⁺D⁻

iii. A⁺B⁺C⁺D⁻/A⁻B⁻C⁻D⁺

iv. A⁻B⁺C⁻D⁺/A⁺B⁻C⁺D⁺