STUDENT POST/E coli strain 146 -> Lactose Metabolism?

John Patrick Latino smakk at
Sat Jan 29 20:13:15 EST 1994

  An individual colony of E. coli 146 (deletion of gpt-lac), carrying a plasmid
with genotype lac I-, Z-, pro+, was streaked onto a minimal lactose plate
(lactose as sole carbon source). Presumably, it should not have formed any
colonies, but in my lab group's case, it did (after incubation @ 37 deg.C for
48 hours. My question is: Since our strain was incapable of utilizing lactose
because of a chromosomal deletion _and_ an amber base pair substitution in the
episome, could the growth on lactose be due to a selection for revertants in
the amber mutation?

 We also plated colonies of the same strain on different
media, including minimal glucose+X-Gal (which turns blue in the presence of 
B-galactosidase). In those cases, *white* colonies were observed. I would think
that if the colonies on the lactose plates were revertants, then revertants
would _also_ occur on the X-Gal plates, revealing blue colonies.

Any e-mailed or posted responses/input on this dilemma would be much ap-
preciated, as we have exhausted our resources (our biochem testbook was of no
use whatsoever). Thanks!   

-John Latino
-Kristina Yu

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