Isolation and analysis of two Escherichia coli K-12 ilv attenuator deletion mutants with high-level constitutive expression of an ilv-lac fusion operon

The absence of branched-chain amino acid and growth rate control at the internal ilvEp promoter of the ilvGMEDA operon

The question of whether the promoter ilvEp, located in the coding region of ilvM, the second structural gene in the ilvGMEDA operon, is subject to either amino acid- or growth rate-mediated regulation is examined. The experiments described here were performed with ilvEp-cat and ilvEp-lac fusions carried as single copies on the chromosome. The activity of the ilvEp promoter was found to respond neither to the availability of branched-chain amino acids nor to a wide range of growth rates between 35 to 390 min. In the absence of any known role for the products of the ilvGMEDA operon when repressing levels of branched-chain amino acids are present, there appears to be only a gratuitous role for the transcription at ilvEp.

Specificity of attenuation control in the ilvGMEDA operon of Escherichia coli K-12

Three different approaches were used to examine the regulatory effects of the amino acids specified by the peptide-coding region of the leader transcript of the ilvGMEDA operon of Escherichia coli K-12. Gene expression was examined in strains carrying an ilvGMED'-lac operon fusion. In one approach, auxotrophic derivatives were starved of single amino acids for brief periods, and the burst of beta-galactosidase synthesis upon adding the missing amino acid was determined. Auxotrophic derivatives were also grown for brief periods with a limited supply of one amino acid (derepression experiments). Finally, prototrophic strains were grown in minimal medium supplemented with single and multiple supplements of the chosen amino acids. Although codons for arginine, serine, and proline are interspersed among the codons for the three branched-chain (regulatory) amino acids, they appeared to have no effect when added in excess to prototrophs or when supplied in restricted amounts to auxotrophs. Deletions removing the terminator stem from the leader removed all ilv-specific control, indicating that the attenuation mechanism is the sole mechanism for ilv-specific control.

Mutations replacing the leucine codons or altering the length of the amino acid-coding portion of the ilvGMEDA leader region of Escherichia coli

The specificity of regulation by attenuation of the ilvGMEDA operon of Escherichia coli was examined by making alterations in the peptide-coding portion of the leader region. The effects of the alterations on attenuation control were monitored by operon fusions with the lacZ or cat gene. Substitution of the tandem leucine codons with arginine codons did not result in arginine control of attenuation even though the altered leader transcripts contained three consecutive arginine codons. Substitution of the single leucine codon with a proline codon at position 10 of the putative peptide, which had been shown to be important in the regulation of the Serratia marcescens ilv operon, did not result in control of attenuation by proline. Since the formation of neither proline nor arginine biosynthetic enzymes is regulated by attenuation control, the effect of tandem phenylalanine codons in place of the tandem leucine codons was examined and found not to result in control by phenylalanine supply. The latter failure may have been due to a configuration in the secondary structure of the protector stem of the leader transcript different from that of the wild-type transcript. The results of the study favored the idea that the lead ribosome does not initiate translation of the leader transcript until after the RNA polymerase has reached the pause site (117 bases into the leader region).

Analysis of regulation of the ilvGMEDA operon by using leader-attenuator-galK gene fusions

Five of the genes for the biosynthesis of isoleucine and valine form the ilvGMEDA operon of Escherichia coli K-12. Expression of the operon responds to changes in the availability of isoleucine, leucine, and valine (ILV). Addition of an excess of all three amino acids results in reduced expression of the operon, whereas limitation for one of the three amino acids causes an increase in expression. The operon is preceded by a leader-attenuator which clearly regulates the increased expression that occurs due to reduced aminoacylation of tRNA. To assess the factors that result in the reduced expression of this operon upon the addition of ILV, a series of plasmids were constructed in which the ilv regulatory region was fused to galK. In response to addition of the amino acids, expression of the galK gene fused to the leader-attenuator decreased five- to sevenfold, instead of the twofold observed for the chromosomal operon. A deletion analysis with these plasmids indicated that the ILV-specific decrease in expression required an intact leader-attenuator but not ilvGp2 or the DNA that precedes this promoter. This conclusion was supported by both S1 nuclease analysis of transcription initiation and determination of galK mRNA levels by RNA-RNA hybridization.

Point mutations in the regulatory region of the ilvGMEDA operon of Escherichia coli K-12

The ilvGMEDA operon of Escherichia coli K-12 is preceded by a regulatory region containing a promoter, a leader, and an attenuator. This region has been extensively characterized biochemically. In this note point mutations of the regulatory region are reported. The effect of these mutations on expression from the ilv regulatory region supports the previous biochemical analysis.

Translational coupling between the ilvD and ilvA genes of Escherichia coli

The hypothesis that translation of the ilvD and ilvA genes of Escherichia coli may be linked has been examined in strains in which lacZ-ilvD protein fusions are translated in all three reading frames with respect to ilvD. In these strains, the nucleotide sequence was altered to obtain premature termination of ilvD translation, and in one strain translation termination of ilvD DNA occurred two bases downstream of the ilvA initiation codon. In the wild-type strain, the ilvD translation termination site was located two bases upstream of the ilvA start codon. In each of the mutant strains, expression of ilvA, as determined by the level of threonine deaminase activity, was strikingly lower than in the wild-type strain. The data suggest that expression of ilvD and ilvA is translationally coupled. By inserting a promoterless cat gene downstream of ilvA, it was shown that the differences in enzyme activity were not the result of differences in the amount of ilvA mRNA produced.

Role of codon choice in the leader region of the ilvGMEDA operon of Serratia marcescens

Leucine participates in multivalent repression of the Serratia marcescens ilvGMEDA operon by attenuation (J.-H. Hsu, E. Harms, and H.E. Umbarger, J. Bacteriol. 164:217-222, 1985), although there is only one single leucine codon that could be involved in this type of control. This leucine codon is the rarely used CUA. The contribution of this leucine codon to the control of transcription by attenuation was examined by replacing it with the commonly used leucine codon CUG and with a nonregulatory proline codon, CCG. These changes left intact the proposed secondary structure of the leader. The effects of the codon changes were assessed by placing the mutant leader regions upstream of the ilvGME structural genes or the cat gene and measuring acetohydroxy acid synthase II, transaminase B, or chloramphenicol acetyltransferase activities in cells grown under limiting and repressing conditions. The presence of the common leucine codon in place of the rare leucine codon reduced derepression by about 70%. Eliminating the leucine codon by converting it to proline abolished leucine control. Furthermore, a possible context effect of the adjacent upstream serine codon on leucine control was examined by changing it into a glycine codon.

Effects of deletion and insertion mutations in the ilvM gene of Escherichia coli

A plasmid was constructed that carried the ilvG and ilvM genes and the associated promoter and leader regions derived from the K-12 strain of Escherichia coli. The ilvG gene contained a + 1 frameshift mutation that enabled the plasmid to specify acetohydroxyacid synthase II. The plasmid was modified by deletions in the terminus of and within the ilvM gene and by insertions into the ilvM gene. The effects of these modifications on the phenotypes of the plasmids were examined in a host strain that lacked all three isozymes of acetohydroxyacid synthase. Most of the ilvM mutant plasmids so obtained permitted growth of the host strain in the absence of isoleucine but not in the absence of valine. Growth in the presence of valine, however, was very slow. No significant acetohydroxyacid synthase activity could be detected even when the cells were grown in a valine-supplemented minimal medium. It thus appears that, at most, only a very low level of acetohydroxyacid synthase activity occurred with ilvG in the absence of ilvM and that low activity was more effective for acetohydroxy butyrate formation than for acetolactate formation. The ilvM gene product could be formed under the control of the lac promoter in the presence of a plasmid that carried an in-frame gene fusion between lacZ and the downstream portion of ilvG. Extracts from the host strain that contained such an IlvG(-)-IlvM+ plasmid could be combined with extracts from cells that contained one of the IlvG+-IlvM- plasmids to yield acetohydroxyacid synthase activity. Thus, the ilvM and ilvG genes could be expressed independently of each other.

Leucine regulation of the ilvGEDA operon of Serratia marcescens by attenuation is modulated by a single leucine codon

The effect of leucine limitation and of restricted leucine tRNA charging on the expression of the ilvGEDA operon of Serratia marcescens was examined. In this organism, the ilv leader region specifies a putative peptide containing only a single leucine codon that could be involved in leucine-mediated control by attenuation (E. Harms, J.-H. Hsu, C. S. Subrahmanyam, and H. E. Umbarger, J. Bacteriol. 164:207-216, 1985). A plasmid (pPU134) containing the DNA of the S. marcescens ilv control region and three of the associated structural genes was studied as a single chromosomal copy in an Escherichia coli strain auxotrophic for all three branched-chain amino acids. The S. marcescens ilv genes responded to a multivalent control similar to that found in other enteric organisms. Furthermore, the S. marcescens ilv genes were derepressed when the charging of leucine tRNA was restricted in a leuS derivative of E. coli that had been transformed with pPU134. It was concluded that ribosome stalling leading to deattenuation is not dependent on either tandem or a consecutive series of codons for the regulatory amino acid. However, the fact that the single leucine codon is a less frequently used codon (CUA) may be important. The procedure for obtaining the cloned ilv genes in single chromosomal copy exploited the dependence of ColE1 replicons on the polA gene. The cloning experiments also revealed a branched-chain amino acid-glutamate transaminase in S. marcescens that is different from transaminase B.

Internal promoter in the ilvGEDA transcription unit of Escherichia coli K-12

Segments of the ilvGEDA transcription unit have been cloned into the promoter tester plasmid pMC81. This vector contains cloning sites situated upstream of the lacZ gene coding for beta-galactosidase. Using this method we have quantitatively evaluated in vivo (i) the activity of previously described promoter, pG, preceding ilvG; (ii) the relative activity of pE promoter, previously postulated to be located between ilvG and ilvE; and (iii) the effect of the frameshift site present in the wild-type ilvG gene by comparison with mutant derivatives lacking this frameshift site. Isogenic derivatives of strain MC1000 were constructed by transduction with phage P1 grown on rho-120, delta(ilvGEDA), delta(ilvED), and ilvA538 hosts. The potential effects of these alleles that were previously postulated to affect ilvGEDA expression were assessed in vivo by monitoring beta-galactosidase production directed by ilv DNA fragments. Cloned ilv segments were also tested for activity in vitro with a DNA-directed coupled transcription and translation system. The production in vitro of ilv-directed ilv gene expression and beta-galactosidase expression with ara-ilv-lac fusions paralleled the in vivo activity.

New regulatory genes involved in the control of transcription initiation at the thr and ilv promoters of Escherichia coli K-12

The gene ileR+, considered to encode a transacting protein involved in the regulation of the thr and ilv operons of Escherichia coli, has been cloned and localized to a 1.2 Kb BglII-SalI fragment of DNA. In strains harboring attenuation-defective fusions of lacZ to the promoter regions of the thr and ilv operons, ileR mutations lead to beta-galactosidase levels higher than those of the deattenuated parental strains. Reduced utilization of the thr and ilv promoters was observed in ileR cells harboring either ilvR+ plasmids or plasmids leading to the hyperproduction of Trp repressor. These results support the idea that ileR+ encodes a repressor protein that negatively affects the expression of the thr and ilv operons. Two additional trans-acting positive regulatory elements that act at the thr and ilv promoters have been identified by an analysis of deletion mutants. It thus appears that there exist positive as well as negative controlling elements that can act independently of attenuation to modulate the ilv and thr operons.