Duplication of a gene belonging to an arginine operon of Escherichia coli K-12.

MONG the structural determinants of the enzymes involved in arginine bioA synthesis in Eschi ch ia codi, four are clustered in the order argE, argC, argB, argH, and probably adjacent (GLANSDORFF 1965, Figure I ) ; the synthesis of enzyme E is however not strictly coordinated with the synthesis of enzymes C, B and H (BAUMBERG, BACON and VOGEL 1965; GLANSDORFF and SAND 1965). The expression of the 3 latter loci has been found polarized from left to right (GLANSDORFF and SAND 1965, SAND and GLANSDORFF 1967), the fate of argE giving rise to conflicting interpretations (ibid., GLANSDORFF 1967, VOGEL et al. 1967). More recent work supports the view that the cluster contains 2 successive operons, both polarized from left to right: argE and argC, B, H (CUNIN et al. 1968, SAND et al. in preparation). A cell harbouring a strongly polar argC or argB mutation has its potential of argininosuccinase (enzyme H) synthesis drastically lowered; as a consequence it grows very slowly in minimal medium supplemented with ornithine. This property was used to investigate by which ways a cell could recover from the polar effect of a mutation without being forced to revert to wild phenotype. Ornithine utilizers (Om-ut) , but still A r g derivatives of polar mutant argB2 were found to occur spontaneously, at a frequency of about low5, in sharp contrast with the frequency of reversion to wild phenotype (IO-lO). One of these strains (MN8C), growing as well on ornithine as the non-polar argB mutants, was studied extensively. This communication adds further experimental support to the conclusion reached in a preliminary account of this work (GLANSDORFF 1966), namely that MN8C carries a duplication of argH.