Pseudoinversions in the Chromosome of Escherichia Coli

HE following observations are mainly methodological; they concern the use of bacterial conjugation in genetic topography. However, as will be shown hereafter, they are by-products of investigations on the genetic control of arginine biosynthesis in Escherichia coli and have precise (although limited) implications for the treatment of this question. Four of the eight sti-uctural genes of the latter pathway are tightly clustered in the order argE-C-B-H ( GLANSDORFF 1965) ; Figure 1 gives the gene-enzyme correspondance, as defined by Mas , MAAS, WIAME and GLANSDORFF (1964). Both enzymatic determinations and genetic evidence suggest that argB and H (and probably C ) form an operon, the fate of argE being uncertain ( GLANSDORFF and SAND 1965). It was thus expected that chromosomal rearrangements which would modify the spatial relationships of the arg loci should also alter the regulation oi their expression. In this last respect, we became interested by the E. coli strain AB1206, isolated by PITTARD. LOUTIT and ADELBERG (1963) ; this organisrii harbours an episome