Kinetics of Induction of the Lactose Operon on an Episome inSalmonella typhimurium

Abstract

A kinetic study of induction of the enzymes of the lactose operon was carried out under conditions known to affect the kinetics of derepression of the enzymes of the histidine operon. The results show that the lactose system is similar to the histidine system in its responsiveness to conditions thought to affect the formylating capacity of the cell. This was demonstrated in the following ways: (i) trimethoprim, which is known to reduce the formylating capacity of the cell, gives rise to a relatively long interval between the times of induction of 3-galactosidase and transacetylase; (ii) under conditions in which the histidine operon is derepressed, chloramphenicol causes a prolongation of the interval between the times of induction of the two enzymes, and this prolongation is reversed by adenine, methionine, and serine, compounds known to enrich the one-carbon pool of the cell; and (iii) 4-amino-5-imidazolcarboxamide ribonucleoside, a compound which may act as a drain for formyl groups, reverses the effect of the latter compounds. The finding that the interval between the times of induction of the two enzymes is shortened under conditions expected to maintain a relatively high intracellular fo rmylating capacity suggests that under certain conditions translation of the polycistronic messenger ribonucleic acid of the lactose operon may be initiated at more than one site or may proceed more rapidly from the operator end. From previous studies on the kinetics of derepression of the histidine enzymes in a wide variety of histidine auxotrophs of Salmonella typhimurium, it was concluded that derepression may occur either simultaneously or sequentially, depending upon the formylating capacity of the cell (8). The enzymes became derepressed simultaneously in those mutants with an adequate formylating capacity, whereas they became derepressed in a temporal sequence, corresponding to the order of the genes in the histidine operon, in those mutants with a reduced formylating capacity. The reason for the differences in formylating capacity among the various histidine auxotrophs is that those mutants with a block in any of the last four steps of the histidine pathway produce phosphoribosyl aminoimidazole carboxamide (PR-AIC) as a byproduct of the path

Operon Coordination in Different Bacterial Hosts

“…Thus, the level of lac gene products is reduced by catabolite repression (12), polar mutations in structural genes (14), and mutations in the promoter region of lac (9). Ballesteros-Olmo et al (2) have also shown that the methylating capacity of the cell influences the kinetics of induction of the lac operon when it is carried by cells of Salmonella typhimurium, and Fox (6) has demonstrated a lipid requirement for normal induction of the f,-galactoside (lac) permease system in Escherichia coli. This latter activity is dependent upon the membrane-bound M protein (7).…”