Correlation between location and time of expression for genes in a single operon.

When the histidine Operon in Salmonella typhimurium becomes derepressed in the presence of 4-amino-5-imidazole carboxamide ribonucleoside, derepression of the enzymes for histidine biosynthesis Occurs in a temporal sequence which corresponds with the positional sequence of the genes in the histidine Operon, approximately 20 minutes intervening between the derepression of the first and that of the last enzyme studied. In the case of the deletion mutant, hisH,B22, the interval between the derepression of the first and that of the last enzyme was reduced from 20 minutes to 10 minutes. This reduction was due exclusively to the fact that the interval between the derepressions of the two enzymes, the structural genes for which are located on either side of the deletion, was almost completely eliminated. The results support the genetic evidence for the physical absence of genetic material situated between these two genes.