Operon Coordination in Different Bacterial Hosts

Abstract

Coordination of gene expression in the lac operon was compared in Escherichia coil and Salmonella typhimurium as an approach to detecting possible differences in protein synthesis or membrane structure between organisms. Either a wild-type F' lac prOAB episome or the same episome with a polar mutation in one of the lac genes was introduced into pro-derivatives of the two strains of bacteria. Activity assays showed that the (3-galactosidase levels were only slightly lower in the S. typhimurium cells than in E. coli cells, whereas the transacetylase levels were significantly higher in S. typhimurium for all of the lac markers tested. Galactoside transport activities were always comparable in the two strains of bacteria; this latter result indicates that the cell envelopes of E. coli and S. typhimurium do not differ sufficiently to affect the membrane-associated lac transport system. It was found, however, that the specific transport activity is very sensitive to culture age in both bacteria, and decreases rapidly in cultures past the mid-exponential phase of growth.Expression of the lac operon is sensitive to changes in a variety of genetic and physiological conditions in the cell. 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).We propose that another way to examine the effect of cytoplasmic factors on gene expression is to compare the coordination of one operon in different bacterial species. If the genes of the lac operon in S. typhimurium are transcribed into one polycistronic messenger ribonucleic acid (RNA) as they are in E. coli (1), then a comparison of the relative amounts of lac operon proteins made in the two bacteria (when both carry an F'lac episome) should indicate whether genes in the specific messenger are translated with equal efficiencies in the two hosts. The lac permease activities should also reflect any differences in membrane structure between the two strains. There is physical evidence (18), for the R factors at least, that in E. coli and in S. typhimurium there is only one episome copy per chromosome. Some of the previous studies on the expression of genes from E. coli in other bacterial hosts have utilized the episomes F'lIac (2, 3, 5), F'trp (4, 20) and F'pho (19).In the present study, we used an F'lac proAB episome to introduce the lac operon into a lac-pro deletion strain of E. coli and a pro-deletion strain of S. typhimurium, which is normally lac.The f3-galactosidase (z gene product), ,B-galactoside permease (y gene product), and galactoside transacetylase (transacety...

Polycistronic Effects of Catabolite Repression on the lac Operon

“…Such studies also suggest the possibility of other control sites for transcription within the lac operon. It is still unclear why the Z and A cistrons of the lac operon do not produce equimolar amount of protein (34) in Escherichia coli, or why transferring the lac operon of E. coli into Salmonella typhimurium results in a change in the ratio of ,B-galactosidase and transacetylase (31). pg of L-proline, L-tryptophan.…”