Article révisé par les pairs
Résumé : Among the four seemingly adjacent loci of the argECBH cluster of E. coli K-12, the last three are shown to belong to the same unit of coordinated expression; the latter exhibits a clockwise polarity in contrast to all other known E. coli operons, except the cluster governing the synthesis of the pyruvate dehydrogenase complex. The analysis of several deletion and nonsense mutants suggests that argE (the expression of which is not strictly correlated with the functioning of the argCBH group) has the same polarity but is not integrated with the three other genes into one operon. Between polar argC B and B mutants the coefficient of repressibility of enzyme H synthesis varies widely. This feature resembles the reduced repressibility of distal gene activity found in polar mutants in the tryptophan operons of E. coli and S. typhimurium but not in the lac, gal (E. coli) and his (S. typhimurium) operons. Possible implications of the present results and some relevant data that have appeared in the recent literature are discussed. © 1969 Springer-Verlag.