ENZYMATIC-ACTIVITIES IN ULTRAVIOLET-INDUCED DNA-DEGRADATION IN ESCHERICHIA-COLI

Ultraviolet-induced DNA degradation is found to occur in Escherichia coli WP6 polA1 and K-12 RS polA ex1 strains with the former strain degrading its DNA more extensively after 20 J/m2. K-12 SR78 recB21 degrades very little or no DNA after 20 J/m2. At a higher fluence of 60 J/m2, however, about 20% of the DNA is degraded, albeit at a lower rate compared to the wild-type DY998. These data suggest that uvrA-dependent degradation involves the 5'→3' exonuclease activity of polymerase I (exonuclease VI) and recBC exonuclease V activity, with the latter less crucial after exposure to a larger fluence. In WP67 uvrA polA1 strains, uv-induced DNA degradation after 10 J/m2 is abolished unless the cells are incubated in the presence of chloramphenicol. In K-12 DM20 uvrA recB21 strains, post-uv incubation after 10 J/m2 with or without chloramphenicol does not result in DNA degradation, suggesting that chloramphenicol-enhancible degradation involves primarily recBC exonuclease activity.