Abstract
The characterization of two classes of DNA ligase mutants in Escherichia coli is described. The first class consists of three mutations coding for a temperature-sensitive ligase and defines the structural gene for DNA ligase ( lig). The second class of mutants ( lop) overproduces an apparently wild-type enzyme; a genetic diploid analysis implies that these are promoter or operator mutations, lig and lop are cotransduced by phage P1 and map at 46 minutes on the E. coli map. Detailed studies of two lig mutants ( lig4 and lig ts7) are reported, lig ts7 is a conditionally lethal mutation, proving the essential nature of the ligase gene product. Neither mutant has a major defect in recombination or ultraviolet-repair, but both show retarded sealing of 10 S pulse-labeled DNA (Okazaki fragments).
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