A detailed genetic map of the terminus region of the chromosome of Escherichia coli is being constructed. This map will contain the several loci previously known to be near the terminus, and it will also include drug resistance transposons (Tn1, Tn5, Tn10) that have been inserted in this region. The loci will be used to determine precisely where DNA systhesis is inhibited during the replication cycle. This will be determined by DNA-DNA hybridization, using the various transpoon insertions that have been cloned with their flanking DNA. A restriction map of the terminus is also being constructed. Prophage have been inserted into the terminus region, and the chromosomal DNA that is replicated after prophage induction is used for the restriction analysis.