Ditto deletions in the cysC region of the Salmonella chromosome.

PREVIOUS studies have revealed that the cysC region of the Salmonella typhimurium chromosome includes a cluster of five genes arranged in the sequence cysC-D-H-I-J ( MIZOBUCHI, DEMEREC and GILLESPIE 1962). Genes cysC and D are adjacent to each other, as are genes I and J (Figure 1). Genes D and H are separated by a long segment of genetic material for which no functions have been found (the “silent section”). Genes H and I are separated by a similar but shorter section of the approximate length of one gene locus (DEMEREC, GILLESPIE and MIZOBIJCHI 1963). The entire cluster can be carried on a single transducing fragment (MIZOBUCHI et al. 1962). In the cysC region of strain LT-2, deletion mutations occur with an abnormally high frequency. The great majority of these deletions are extensive and are similar to one another (“ditto deletions”) in that their left ends terminate in the silent section and their right ends terminate beyond the cys.7 locus (DEMEREC 1960). Data obtained in preliminary studies on ten ditto deletions indicate that the left ends of the ditto deletions are not distributed at random along the entire silent section but, rather, are restricted to a certain portion of it (DEMEREC et al. 1963). The data in the present paper extend the analysis to 43 additional “ditto deletions” (Ddl) . The Ddl are shown to terminate in a particular segment of the silent section. Points of termination differ slightly among independently isolated Ddl.