Biochemical and regulatory properties of Escherichia coli K-12 hisT mutants

Specific mistranslation in hisT mutants of Escherichia coli

Certain strains of Escherichia coli mistranslate at very high frequencies when starved for asparagine or histidine. This mistranslation is the result of misreading events on the ribosome. The introduction of a hisT mutation into such a strain decreases the frequency of mistranslation during histidine starvation but not during asparagine starvation. The most likely explanation is that the replacement of the pseudouridine residue in the anticodon loop of glutamine specific transfer ribonucleic acid by uridine in hisT mutants leads to an increase in fidelity of transfer ribonucleic acid function. The hisT gene in Escherichia coli has also been more accurately mapped, giving the gene order purF-hisT-aroC-fadL-dsdA.

Strains of Escherichia coli carrying the structural gene for histidyl-tRNA synthetase on a high copy-number plasmid

That portion of the Escherichia coli chromosome carried by a number of lambda transducing phages, all of which carry the gua operon, was mapped using restriction endonucleases. The DNA from one of these transducing phages was subcloned onto pBR322. We have identified two recombinant plasmids which carry the Escherichia coli gene hisS, the structural gene for histidyl-tRNA synthetase. The two plasmids, pSE301 and pSE401, have in common a 3,540 bp fragment of E. coli DNA which is bounded by BglII and SalI restriction endonuclease recognition sites. Strains carrying these plasmids overproduce histidyl-tRNA synthetase 20 to 30 fold. The growth rate of these strains is not affected although the histidine biosynthetic enzymes are derepressed. This derepression seems to be in addition to that caused by introduction of a hisT mutation on the chromosome.

In vivo and in vitro detection of the leader RNA of the histidine operon of Escherichia coli K-12

The DNA of the attenuator region of the histidine operon of Escherichia coli has been transcribed in a purified in vitro system and found to synthesize two major RNA transcripts. The first one, 180 nucleotides long, has been identified as the histidine-specific leader RNA. It contains the coding sequence for the leader peptide [Di Nocera, P. P., Blasi, F., Di Lauro, R., Frunzio, R. & Bruni, C. B. (1978) Proc. Natl. Acad. Sci. USA 75, 4276-4280] and is terminated at the attenuator site. Termination of transcription at this site is extremely efficient in the in vitro system. The leader RNA also has been detected in vivo in a minicell producer strain transformed with plasmids harboring the regulatory region of the histidine operon of E. coli. A second RNA molecule is synthesized in the in vitro system. It has a divergent direction of transcription with respect to the histidine leader RNA, but its role, if any, in the regulation of the histidine operon remains to be ascertained. The existence of the histidine leader RNA lends support to the regulatory mechanism which postulates that regulation of the histidine operon is dependent on the alternative secondary structures that the leader RNA may assume, depending on whether or not the histidine-rich leader peptide is translated.

Transcription-translation coupling and polarity: a possible role of cyclic AMP

Using specific mutants exhibiting altered translational rates we found that when the rate of protein synthesis is reduced the polarity of the lactose and galactose operons is increased. This polarity is in part relieved by cyclic AMP. On the basis of in vitro hybridization experiments were propose that the cyclic AMP-CAP complex and the ribosomes display similar functions, namely destabilization of the RNA-DNA hybrid formed during transcription.

The relA locus and the regulation of lysine biosynthesis in Escherichia coli

The allelic state of relA influences the phenotype of Escherichia coli strains carrying the lysA22 mutation:lysA22 relA strains are Lys- where lysA22 relA+ strains grow (slowly) in the absence of lysine. This physiological effect has been related to an effect of the expression of the relA locus on the regulation of lysine biosynthesis. The fully derepressed levels of some lysine enzymes (aspartokinase III, aspartic semialdehyde dehydrogenase, didhydrodipicolinate reductase) are observed under lysine limitation only in rel+ strains. And the induction of DAP-decarboxylase by DAP is much higher in rel+ than in rel- strains when an amino acid limitation of growth is also realised. These results are in agreement with the hypothesis of Stephens et al. (1975) on a possible role of the stringent regulation as a general signal for amino acid deficiency.

Structural and physiological studies of the Escherichia coli histidine operon inserted into plasmid vectors

A fragment of deoxyribonucleic acid 5,300 base paris long and containing the promoter-proximal portion of the histidine operon of Escherichia coli K-12, has been cloned in plasmid pBR313 (plasmids pCB2 and pCB3). Restriction mapping, partial nucleotide sequencing, and studies on functional expression in vivo and on protein synthesis in minicells have shown that the fragment contains the regulatory region of the operon, the hisG, hisD genes, and part of the hisC gene. Another plasmid (pCB5) contained the hisG gene and part of the hisD gene. Expression of the hisG gene in the latter plasmid was under control of the tetracycline promoter of the pBR313 plasmid. The in vivo expression of the two groups of plasmids described above, as well as their effect on the expression of the histidine genes not carried by the plasmids but present on the host chromosome, has been studied. The presence of multiple copies of pCB2 or pCB3, but not of pCB5, prevented derepression of the chromosomal histidine operon. Possible interpretations of this phenomenon are discussed.

Mutation spoT of Escherichia coli increases expression of the histidine operon deleted for the attenuator

F'-episomes carrying the Salmonella typhimurium wild-type or attenuator-deleted histidine (his) operons were introduced into Escherichia coli strains containing relA or spoT single and double mutations known to affect guanosine 3'-diphosphate 5'-diphosphate (ppGpp) and guanosine 3'-triphosphate 5'-diphosphate (pppGpp) levels. Expression of the his operon and expression of the gene for 6-phosphogluconate dehydrogenase (gnd) were measured during balanced growth in amino acid-rich and minimal media. The data were consistent with the interpretation that ppGpp is a positive effector of his operon expression, whereas pppGpp is not an essential effector. The conclusion that his operon expression is maximally stimulated at a lower than maximum intracellular ppGpp concentration was further confirmed. Neither ppGpp nor pppGpp appeared to influence gnd gene expression. The metabolic regulation of the E. coli his operon was found to be similar to the ppGpp-meidated metabolic regulation of the S. typhimurium his operon.

Effects of altered rho gene product on the expression of the Escherichia coli histidine operon

An altered rho gene product affects expression of the his operon of Escherichia coli. The effect is greater for the operator proximal portion of the his operon than for the operator distal portion. This "rho effect" appears to be independent of the site of action of hisT-altered histidyl-tRNA.

A site of action for tRNA mediated regulation of the ilvOEDA operon of Escherichia coli K12

Transfer RNA (tRNA), rho factor threonine deaminase and the ilvO locus are molecular participants in the regulation of isoleucine-valine (ilv) biosynthesis. Isogenic strains have been constructed with the hisT76 mutation in pairwise combination with ilvO mutations, the rho221 mutation and the ilvDAC115 deletion mutation. The role of the altered tRNA of the hisT76 mutation was found to be independent of the sites of action of the ilvO- mutation, rho factor, and threonine deaminase. The expression of the ilvOEDA operon is stimulated 2-fold when the hisT76 mutation is present in strains containing either ilvO- or rho221 mutations. The expression of the ilvOEDA operon remains nonrepressed in a hisT76 strain deleted for threonine deaminase. These results indicate that the hisT76 undermodified tRNAs are influencing the initiation of transcription of the ilvOEDA operon.

Correlation between the serine sensitivity and the derepressibility of the ilv genes in Escherichia coli relA- mutants

Upon addition of excess one carbon metabolites (including serine)bacteria stop growing because of isoleucine starvation. After such treatment stringent bacteria rapidly resume normal growth whereas relaxed mutants remain unable for some time to grow. We show here that this is due to a lack of derepressibility of ilv genes after the starvation period. Results are also presented which show that RNA polymerase structural mutants may be selected among the clones resistant to a mixture of serine, methionine and glycine, in relA- strains. Finally circumstancial evidence suggests that the one carbon metabolism may be involved in a process controlling isoleucine metabolism.

Nucleotide sequence of the attenuator region of the histidine operon of Escherichia coli K-12

The attenuator region of the histidine operon of Escherichia coli K-12 has a potential coding capacity for two peptides, one of 16 amino acids and another of 30 amino acids. This region is followed by a perfect palindrome of 14 base pairs separated by five nucleotides. A G+C-rich region precedes and follows a possible transcription termination sequence. These features are compatible with a model in which active translation of a leader mRNA interferes with transcription termination, thus causing derepression of the histidine operon. The sequence of the region coding for the hypothetical 16-amino acid peptide is of particular relevance because it indicates the site and a possible mechanism of action of histidyl-tRNAhis in regulating histidine gene expression. Seven contiguous histidine codons are present within this sequence: : formula: (see text)

Expression of the histidine operon in rho mutants of Escherichia coli

Expression of the Escherichia coli histidine operon was measured in four independently isolated sets of strains carrying ten different rho mutations. Rho factor does not act as a major regulatory element of histidine operon attenuation.

Repression of Escherichia coli pyridine nucleotide transhydrogenase by leucine

Addition of 0.1% casein hydrolysate to a minimal growth medium decreased membrane-bound transhydrogenase activity in Escherichia coli by about 80%. Of the amino acids added individually to the growth medium, only leucine and, to a lesser extent, methionine and alanine were effective, alpha-Ketoisocaproate- and leucine-containing peptides repressed the activity, and leucine also repressed activity in adenyl cyclase-deficient and relaxed strains. Derepression of transhydrogenase followed the removal of leucine from the growth medium and was sensitive to rifampin and chloramphenicol. A phosphoglucoisomerase-deficient strain that was forced to use the hexose monophosphate shunt exclusively had normal levels of transhydrogenase, which was repressed by leucine. Transhydrogenase activity doubled in mutants lacking either of the shunt dehydrogenases but was still repressed by leucine. In strains constitutive for the leucine biosynthetic operon, transhydrogenase was repressed by leucine but in strains livR and lst R, with leucine transport resistant to leucine repression, transhydrogenase was not repressed by leucine. These data suggest that transhydrogenase may have a function in the transport of branched-chain amino acids. In a hisT strain (which has altered leucyl-tRNA), transhydrogeanse was at a repressed level without the addition of leucine, suggesting that leucyl-tRNA may be involved in the regulation.

Effect of mutations affecting lysyl-tRNAlys on the regulation of lysine biosynthesis in Escherichia coli

When studying mutants affecting lysyl-tRNA synthetase or tRNAlys (hisT, hisW), a lack of correlation is clearly observed between the amount of lysyl-tRNA and the level of derepression of several lysine biosynthetic enzymes. This exlcudes the possible role of lysyl-tRNA as the specific corepressor of the lysine regulon. However, the level of derepression of DAP-decarboxylase, the last enzyme of the lysine pathway, is very low in the hisT mutant; this indicates that tRNAlys is a secondary effector involved in the regulation of the synthesis of this enzyme.

Promoter- and attenuator-related metabolic regulation of the Salmonella typhimurium histidine operon

Expression of the histidine (his) operon in Salmonella typhimurium was found to be positively correlated with the intracellular level of guanosine tetraphosphate (ppGpp). Limitation for amino acids other than histidine elicited a histidine-independent metabolic regulation of the operon. In bacteria grown at decreased growth rates, his operon expression was metabolically regulated up to a point, after which further decreases in growth rate no longer resulted in further enhancement of operon expression. Studies using strains carrying various regulatory and deletion mutations indicated that metabolic regulation is achieved predominantly by increased RNA chain initiations at the primary (P1) and internal (P2) promoters. Metabolic regulation ordinarly did not involve changes in RNA chain terminations at the attenuator site of the his operon. A model is proposed that involves ppGpp-induced changes in RNA polymerase initiation specificity at particular promoters. A second, special form of metabolic regulation may operate which also is histidine independent, but does involve relief of attenuation.

Escherichia coli mutants with defects in the biosynthesis of 5-methylaminomethyl-2-thio-uridine or 1-methylguanosine in their tRNA

Two tRNA methyltransferase mutants, isolated as described in the accompanying paper (G.R. Björk and K. Kjellin-Stråby, J. Bacteriol. 133:499-207, 1978), are biochemicaaly and genetically characterized. tRNA from mutant IB13 lacks 5-methylaminomethyl-2-thio-uridine in vivo due to a permanently nonfunctional methyltransferase. Thus tRNA from this mutant is a specific substrate for the corresponding tRNA methyltransferase in vitro. In spite of this defect in tRNA, such a mutant is viable. Mutant IB11 is conditionally defective in the biosynthesis of 1-methylguanosine in tRNA due to a temperature-sensitive tRNA (1-methyl-guanosine) methyltransferase. In mutant cells grown at a high temperature, the level of 1-methylguanosine in bulk tRNA is 20% of that of the wild type, demonstrating that in this mutant an 80% deficiency of 1-methylguanosine in tRNA is not lethal. Genetically these two distinct lesions, trmC2, causing 5=methylaminomethyl-2-thio-uridine deficiency, and trmD1, giving a temperature-sensitive tRNA (1-methylguanosine)methyltransferase, are both located between 50 and 61 min on the Escherichia coli chromosome.