- An E. coli strain has metB1 (90 min) and leuA5 (2 min) mutations. It also has strA7 (73 min) mutation and Tn5 transposon which confers streptomycin and kanamycin resistance, respectively, inserted in its chromosome. The mutant strain was crossed with an Hfr strain that is streptomycin sensitive and has a hisG2 mutation (44 min) that makes it require histidine. After incubation for 100 min, the cells were plated on minimal plate supplemented with leucine, histidine and streptomycin to select the metB marker. After purifying 100 of the Met+ transconjugants, one finds that 15 are His+, 2 are Leu+ and 12 are kanamycin sensitive.
The unselected markers are
A. metB1 and leuA5 mutation.
B. leuA5 and Tn5 insertion mutation.
Which of the above statement is correct and what is the position of transposon insertion?
(1) A and before 73 min
(2) B and before 44 min
(3) B and before 73 min
(4) A and before 44 minCore Answer
The correct option is (3) B and before 73 min. Unselected markers are leuA5 mutation and Tn5 insertion (KanR loss in 12/100 = 88% KanS), as they show high counterselection (low inheritance from recipient); Tn5 lies before strA7 (73 min) on Hfr transfer path, co-inherited unless proximal crossover disrupts it.
Experiment Setup
Recipient F- strain carries metB1 (90 min), leuA5 (2 min), strA7 (73 min, streptomycin resistance), Tn5 (KanR, unknown position). Donor Hfr strain is streptomycin sensitive (strA+), hisG2 (44 min, His-). Mating for 100 min allows full chromosome transfer (E. coli map ~100 min). Plating on minimal + leucine + histidine + streptomycin selects Met+ recombinants (metB+ from Hfr, strA from recipient counterselects donor). Of 100 Met+ exconjugants, 15% His+ (hisG+ from Hfr), 2% Leu+ (leuA+ from Hfr), 12% KanS (Tn5 lost, recipient kanS phenotype).
Data Analysis
High His+ (15/100) indicates hisG (44 min) near metB (90 min), transferred and recombined frequently. Low Leu+ (2/100) shows leuA (2 min) distal or counterselected. High KanS (88%) means Tn5 rarely co-inherited with metB+, so Tn5 is proximal to metB on Hfr transfer path (early entry, disrupted by recombination). Unselected markers are recipient traits retained: leuA5 (98% retained) and Tn5 (88% retained, dominant KanR lost only on deletion). MetB1 selected, strA7 counterselected.
Option Analysis
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(1) A and before 73 min: Incorrect. A (metB1, leuA5) wrong; metB1 selected (not unselected), leuA5 alone insufficient. Tn5 position irrelevant here.
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(2) B and before 44 min: Incorrect. B (leuA5, Tn5) correct as unselected, but Tn5 before hisG (44 min) contradicts 88% KanS (proximal markers co-inherit more with distal metB); should be before strA (73 min).
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(3) B and before 73 min: Correct. LeuA5/Tn5 retained highly; Tn5 proximal to metB but before strA7 (73 min) on Hfr path (0-proximal…Tn5…73 strA…44 his…90 metB), explaining high KanS via early recombination.
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(4) A and before 44 min: Incorrect. A wrong (metB1 selected); position mismatches data.
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