Seeking wild-type E. coli strain

[Roberto Albinoni]

I would like to isolate RNA from a wild-type E. coli strain which
lacks lambda or the F factor.

Jensen has shown that the commonly used strains W3110 and MG1655,
as well as their parent, W1485, have a frameshift mutation, and god
knows what else has yet to be detected. (J. Bact. 175, 3401)

Does anyone know of a K-12 strain which was cured of lambda and 
F by less mutagenic methods than those used to cure the ancestors
of the commonly used strains listed above?  Thanks.  Berto.