[Mutation affecting the promotor region of the Escherichia coli K-12 deo-operon]

It was shown that the strain SA1030 (Das et al., 1976) contains a mutation (deoPx), which decreases deoR regulated expression of the deo operon. Thymine dependent strain harbouring deoPx mutation decreases requirement in thymine up to 8-10 microgram/ml. The deoPx mutation causes 9 fold and 3 fold decrease in the activity of thymidine phosphorylase and purine nucleoside phosphorylase, respectively in cytR-deoR- strains, i.e. under the conditions of maximal derepression of the operon. The presence of cya mutation in deoPx background causes reduction of the activity of both enzymes to basal level, no matter whether cytR or deoR repressor proteins are present or not. It is supposed that deoPx mutation blocks the activity of the deoP promoter, while the cytP promoter remains unchanged. On the other hand, the deoR mutation gives rise to a rather high level of the activity of deo enzymes in cya+cytR+ strains harbouring deoPx mutation, as compared to those found in the corresponding deoR+ strains. These data may be explained by the conception that two repressor proteins function in a cooperation with respect to repression of the deo-genes (Hammer-Jespersen, Munch-Petersen, 1975). The deoPx mutation is cotransducible (approximately 50%) with the thr gene and is located near the deo operon to the left of dra, indicating the order of markers on the chromosome to be deoPx--dra--thr. In transduction and conjugation matings deoPx mutation is characterized by a very low frequence or the absence of integration into the chromosomes of some recipient strains. These data, suggest that deoPx mutation is the result of a rearrangement of genetic material in the promoter region of the deo operon.