TABLE 5.

Effect of cro-z8 mutation on P_R expression and on stability of λprm240 under various growth conditions

Medium	Growth temp (°C)	β-Galactosidase activitiesa			Phage yield (phage/cell)b
		PR+	PR+cro-z8	Ratio	λprm240	λprm240 cro-z8	Ratio
Tryptone	30	2,304	714	0.31	0.08	0.008	10
	37	3,869	1,268	0.33	1.3	0.2	6.5
LB	30	2,203	730	0.33	1.8	0.37	4.8
	37	3,491	872	0.25	17	7	2.3

^aFor the β-galactosidase assays, reporters carried the prm240 allele and either the wild-type or cro-z8 mutant allele; the presence of the prm240 allele made little or no difference in the activity of P_R under these conditions (not shown). We surmise that the effect of the cro-z8 mutation differed from the lower value (11%) found when Cro was expressed from the lac promoter on a plasmid (46) due to differences in the 5′ leader of the mRNA. Since P_R was more active at 37°C than at 30°C, both in tryptone broth and in LB, direct comparisons between results at different temperatures are not meaningful, since the level of Cro should also be somewhat higher in the cro-z8 lysogen at 37°C.

^bFor measurement of free phage levels, as part of the experiment in Table 4 the λprm240 cro-z8 lysogen JL5016 was grown in parallel with JL6112. Data for λ prm240 phage yields are from Table 4.