TABLE 5.

CI⁺ limitsimmλ rescue in Y836 at 39° following λimm434 infection

	Cell viability TB + Amp/TBa			Recombination frequency (×10−5)c
Host Y836[pCI+]a	30°	39°	Colonies with plasmid loss at 39°b	30°	39°
Experiment 1	0.91	1.05	0/220	<0.3	0.5
Experiment 2	1.04	1.02	0/220	<0.1	1.4

Plasmid pCI⁺ is multicopy and expresses the cI⁺ gene encoding CI repressor of λ (see materials and methods).

^aSingle colonies of Y836[pCI⁺] were prepared on TB + Ampicillin (100 μg/ml) agar plates, and a single colony was used to inoculate a culture of TB + Amp (100 μg/ml). Cell aliquots were infected and inoculated into TB (Experiment 1) or TB + Amp (100 μg/ml) (Experiment 2) for 90 min and the recombination frequency was determined. The same cell culture was subsequently plated on TB and on TB + Amp plates and incubated at 30° or 39°; the cell titer on TB + Amp plates was divided by the cell titer on TB plates. The growth of culture cells on TB plates does not require that cells maintain plasmid.

^bSingle colonies on TB plates incubated at 39° were stabbed to TB + Amp and TB plates, respectively, for measuring the proportion of Y836 cells that had lost pCI⁺. In Experiment 1, all colonies from two dilution plates were picked.

^cThe frequency of immλ recombinants from the crosses were determined by dividing the PFU titer on TC600[pRP42] cells by the PFU titer on TC600 cells.