DNA in the terminal region of the chromosome of was labelled by a procedure in which cells in sporulation inducing conditions were pulse-labelled with [H]thymidine and then treated with -hydroxyphenylazouracil, an inhibitor of DNA synthesis. The labelled DNA in isolated spores yielded a small number of restriction fragments. About 14 RI fragments with a total length of 80 kb were labelled in a 2.5 min pulse. A fragment of 4.0 kb had the highest specific radioactivity in terminally labelled DNA from several strains. One of these strains lacked the 120 kb prophage of SPβ that is normally integrated close to the terminus. Loss of the 120 kb prophage did not affect the point of termination which must therefore be regarded as a specific ‘stop’ sequence. Labelled terminus DNA has been used to identify lambda (Charon 4A) clones containing sequences derived from the terminal region. The total length of the restriction fragments present was 150 kb and adds another 90 kb to the 150 kb region mapped previously. Only one group of these sequences was present in a strain (CU1695) carrying a deletion spanning from SPβ to the right of the terminus and This suggests that the terminator sequence found in the wild-type can be deleted but presumably this strain has an alternative mechanism of termination.


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