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SUMMARY The standard methods of scoring for unselected markers the recombinant clones arising from crosses between various substrains of Escherichia coli K-12 do not easily allow recognition of types intermediate between or slightly deviating from the parental phenotypes. The theories of Hinshel- wood and his school suggest the general possibility of a continuous series of such intermediates, which have in fact been reported among the progeny of certain drug-resistant x drug-sensitive crosses. To examine whether similar types result with other kinds of unselected marker, crosses were performed in which the parents differed in their ability to utilize lactose (lac character) and/or to grow in the absence of proline (pro character), as well as in other characteristics utilizable as selected markers. Growth of recombinant and parental isolates on minimal media containing lactose as carbon source, or lacking proline, was then characterized quantitatively. No recombinants with intermediate pro character were found, and only one with intermediate lac character. However, in some cases the recombinants differed in lac or pro character from either parent in other ways. One donor strain, proline-requiring (pro-) and unable to utilize lactose (lac-), yielded pro- and lac- recombinants whose eventual growth in proline- less and lactose media respectively was greatly retarded by comparison with the donor parent. This donor strain was also crossed with a pro+ lac+ recipient which had been ' trained ' to grow in minimal lactose medium with optimal growth rate. In lactose media, lac+ recombinants all grew at the optimal rate, while Zac- recombinants showed growth still retarded as compared to the Zac- parent, but considerably more rapid than similar recombinants from the cross involving an untrained lac- recipient.

A Quantitative Study of the Expression of Unselected Lactose and Proline Markers in Escherichia coli K-12 Recombinants and in the Parent Strains