A. A scheme of the possible futile DNA-rU misincorporation/excision cycle. B. TLD kinetics of the dinB thyA and umuCD thyA mutants. The strains are: thyA, KKW58; dinB thyA, KJK90; umuCD thyA, KJK87; dinB umuCD thyA, RA36. C. TLD kinetics of the rnhA thyA and rnhB thyA mutants. The strains are: thyA, KKW58; rnhA thyA, RA33; rnhB thyA, RA34; rnhAB thyA, RA35. D. A representative gel (1.1% agarose) of DNA-rN density determination in plasmid DNA (plasmid is pEAK86) isolated from rnhB mutant cells. +dT, growth in the presence of thymidine; –dT, starvation without thymidine. RNase HII, in vitro treatment with RNase HII. Strains are: rnhB, RA34; rnhAB, RA35. E. DNA-rN density (presented as frequency = 1/density) in the rnhB and rnhAB mutants grown in the presence of absence of dT for 5 hours, from several gels like in “E”. F. Evolution of the chromosomal DNA amount in the thyA (KKW58) versus rnhAB thyA (RA35) mutants during dTTP starvation. G. TLD kinetics of the dinB rnhAB thyA and umuCD rnhAB thyA mutants. The strains are: rnhAB thyA, RA35; umuCD rnhAB thyA, RA40; dinB rnhAB thyA, RA39; dinB umuCD rnhAB thyA, RA41.