The trpA, cysB and purU genes are genetically linked. A wild-type trpA gene is
required for growth on media lacking tryptophan; a wild-type cysB is required for
growth on media lacking cysteine; and a wild-type purU gene is required for
growth on media lacking purines.
To map relative locations of trpA, cysB, and purU, the generalised transducing
bacteriophage P1 was grown on a trpA+ cysB+ purU- strain, and the resulting
lysate was used to infect a trpA- cysB- purU+ strain. 500 Trp+ colonies were
selected and screened for cotransduction of the cysB+ and purU- alleles. In
addition, 500 Cys+ colonies were selected and screened for cotransduction of
the trpA+ and purU- alleles.
Please see attached file for properly formatted question, table, and formulas.
a) What growth medium would you use to select for Trp+
transductants (the first
column)?
b) What is the co-transduction frequency for: (i) trpA+ and cysB+, (ii) trpA+ and
purU-
, and (iii) cysB+ and purU-?
c) What is the distance in minutes between: (i) trpA and cysB, (ii) trpA and purU,
and (iii) cysB and purU?
d) Draw a map showing the order and relative positions of the three genes.
e) Briefly explain why there are so few trpA- purUtransductants when Cys+
colonies are selected (the second column).
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