Summary: Thermosensitive mutants of deficient in peptidoglycan synthesis were screened for mutations in the -diaminopimelate (-Apm) metabolic pathway. Mutations in two out of five relevant linkage groups, and , were shown to induce, at the restrictive temperature, a deficiency in -Apm synthesis and accumulation of UDP-MurNAc-dipeptide. Group is heterogeneous; it encompasses mutations that confer deficiency in the deacylation of -acetyl--Apm and accumulation of this precursor. Accordingly, these mutations are assigned to the previously identified locus . Mutations in linkage group entail a thermosensitive aspartokinase I. Therefore, they are most likely to affect the structural gene of this enzyme, which we propose to designate . Mutation , previously reported to affect the pyruvate carboxylase, was shown to confer a deficiency in aspartokinase I, not in the carboxylase, and to belong to the locus. is closely linked to , the putative gene of dipicolinate synthase. In conclusion, mutations affecting only two out of eight steps known to be involved in -Apm synthesis were uncovered in a large collection of thermosensitive mutants obtained by indirect selection. We propose that this surprisingly restricted distribution of the thermosensitive mutations isolated so far is due to the existence, in each step of the pathway, of isoenzymes encoded by separate genes. The biological role of different aspartokinases was investigated with mutants deficient in and genes. Growth characteristics of these mutants in the presence of various combinations of aspartate family amino acids allow a reassessment of a metabolic channel hypothesis, i.e. the proposed existence of multienzyme complexes, each specific for a given end product.


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