+dT, growth in the presence of thymidine; –dT, starvation without thymidine. Strains are: thyA, KKW58; ung thyA, KJK78; dut ung thyA, RA18. A. A representative gel (1.1% agarose) of DNA-dU density determination in plasmid DNA (pMTL20). UDG, treatment with uracil-DNA glycosylase; Exo III, treatment with exonuclease III; U + E, treatment with both UDG and Exo III. B. Quantification of the DNA-dU density (presented as frequency = 1/density) in the ung thyA mutant grown in the presence of absence of dT, from several gels like in “A”. The “–dT” culture is processed 5 hours after dT removal by filtration. There are two different conditions of growth in the presence of dT, though: “+dT #1” is also processed 5 hours after the filtration (but dT was re-added in this case), so the culture becomes stationary. In contrast, “+dT #2” is processed when the culture reaches OD=0.6. C. A representative gel (3% alkaline agarose) of DNA-dU density determination in a 234 nt long fragment of pBR322. D. Quantification of the DNA-dU density (presented as frequency, = 1/density, and in the same scale as in “B”, for direct comparison) in the dut ung thyA mutant grown in the presence of absence of dT for 5 hours, from several gels like in “C”.