14. Phages are collected from an infected E. coli donor strain of genotype cys⁺ Leu⁺ thr⁺ and used to transduce a recipient of genotype cys^– Leu^– thn^–, The treated recipient population is plated on a minimal medium supplemented with leucine and threonine. Many colonies grew. Which one of the following combination of genotypes are appropriate for the colonies that grew?
(1) cys⁺ leu⁺ thr⁺, cys⁺ leu^– thr⁺, cys^– leu⁺ thr^–
(2) cys^– leu⁺ thr⁺, cys⁺ leu^–thr^–, cys⁺leu⁺ thr^–
(3) cys^– leu^–the^–, cys^– leu^– thr⁺, cys^– leu⁺ thr^–
(4) cys⁺ leu^– the^–, cys⁺ leu^– thr⁺, cys⁺ leu⁺ thr^–

Introduction
Phage transduction in E. coli uses phages from a cys+ leu+ thr+ donor strain to transfer DNA to a cys- leu- thr- recipient, enabling prototrophic growth on selective media. When plated on minimal medium supplemented with leucine and threonine, only colonies able to synthesize cysteine (cys+) can grow, as leucine and threonine are provided but cysteine requires transduction from the donor. This setup selects for various cys+ recombinants, making it key for CSIR NET Life Sciences genetics preparation.

Medium Selection Principle

Minimal medium lacks cysteine, so recipients remain auxotrophic (cys-) and cannot grow without supplementation. Adding leucine and threonine supports leu- thr- strains if they acquire cys+, but untransduced cys- leu- thr- cells fail due to cysteine starvation. Colonies arise solely from transduction events yielding cys+ genotypes via phage-mediated DNA fragments integrating into the recipient chromosome.

Option Analysis

• Option (1) cys+ leu+ thr+, cys+ leu- thr+, cys- leu+ thr-: First two genotypes grow (both cys+ use supplied leu/thr as needed); third (cys-) cannot synthesize cysteine and dies.

• Option (2) cys- leu+ thr+, cys+ leu- thr-, cys+ leu+ thr-: First (cys-) fails despite supplements; second and third grow as cys+.

• Option (3) cys- leu- thr-, cys- leu- thr+, cys- leu+ thr-: All cys- and cannot grow on cysteine-free medium.

• Option (4) cys+ leu- thr-, cys+ leu- thr+, cys+ leu+ thr-: All cys+ and grow using leucine/threonine supplements.

Correct Answer

Option (4) lists only viable cys+ leu- thr-, cys+ leu- thr+, and cys+ leu+ thr- genotypes that form colonies, as all can synthesize cysteine while using provided leucine and threonine. This reflects generalized transduction outcomes where single, double, or triple markers transfer based on phage packaging.