Restriction endonuclease mapping of the Escherichia coli K12 chromosome in the vicinity of the ilv genes

Organization of the early region of bacteriophage phi 80. Genes and proteins

An EcoRI segment containing the early region of bacteriophage phi 80 DNA that controls immunity and lytic growth was identified as a segment whose presence on a plasmid prevented growth of infecting phi 80cI phage. The nucleotide sequence of the segment (EcoRI-F) and adjacent regions was determined. Based on the positions of amber mutations and the sizes of some gene products, the reading frames for five genes were identified. From the relative locations of these genes in the genome, the properties of some isolated gene products, and the analysis of the structures of predicted proteins, the following phi 80 to lambda analogies are deduced: genes cI and cII to their lambda namesakes; gene 30 to cro; gene 15 to O; and gene 14 to P. An amber mutation by which gene 16 was defined is a nonsense mutation in the frame for gene 15 protein, excluding the presence of gene 16. An amber mutation in gene 14 or 15 inhibits phage DNA synthesis, as is the case with their lambda analogues, gene O or P. Some characteristics of proteins from the early region predicted from their primary structures and their possible functions are discussed.

Transposition of Tn1000: in vivo properties

Transposition mediated by the Tn1000 transposase was investigated by using transposon variants carrying synthetic or wild-type termini but no intact Tn1000 genes. Transposon Tn1001, whose only homologies to Tn1000 are in its 38-base-pair terminal inverted repeats, transposed at the same rate as Tn1005, an artificial construct carrying wild-type Tn1000 termini and approximately 1 kilobase of flanking Tn1000 DNA at each end, when transposase was supplied in trans. The majority of the transpositions into pOX38 gave rise to cointegrates, but approximately 10% of the products expressed phenotypes of direct transpositions. The expression and temperature dependence of the tnpA gene product were examined by studying transposition of Tn1001 to bacteriophage lambda. The temperature optimum for transposition was 37 degrees C, and the transposase was stable for up to 2 h at this temperature.

Interaction of cyclic AMP receptor protein with the ilvB biosynthetic operon in E. coli

DNase and restriction site protection studies show that cAMP and its receptor protein (CRP) bind to the promoter of the ilvB operon at approximately position -44 to -82. This region contains sequences that are homologous to those found in other CRP-dependent promoters. In vitro transcription from the ilvB promoter was markedly increased by the addition of cAMP and CRP. This stimulation was not found when the ilvB template lacked the proposed CRP binding site. cAMP-CRP did not alter the extent of transcription termination within the ilvB leader suggesting that this regulatory system may be independent of the attenuation mechanism involved in the negative control of this operon. The results of restriction enzyme site protection studies and experiments with altered promoter fragments indicate that the mechanism for CRP stimulation of the ilvB operon may be similar to a model recently proposed for lac.

Genetics of bacteriophage phi 80--a review

The genetic maps of bacteriophage lambda and lambdoid phage phi 80 are compared. The gene organization of phi 80 is very similar to that of lambda, as shown by isolation and characterization of many am, ts and c (clear) mutants of the phage. In general, the essential genes located in the same position on the genetic map of the phages lambda and phi 80 fulfill the same functions. These include the gene clusters coding for the head and tail proteins, genes for DNA synthesis, and the genes controlling lysogeny and late gene expression. The specific regulatory features of phi 80 in relation to the N function of lambda are discussed, but they require further clarification. The two phages differ in immunity specificity, host range, conversion property and temperature sensitivity.

Cloning of the pif region of the F sex factor and identification of a pif protein product

This paper reports a detailed investigation of the pif region of the F factor responsible for inhibition of development of T7 and related "female-specific" phages. We have mapped a series of pif::Tn5 insertions to a region between 39.6 and 42.8 kilobases on the physical map of F. All pif::Tn5 insertions plated T7 at full efficiency; most were clustered in a 1.8-kilobase interval on both sides of the EcoRI site located at F coordinate 40.3 kilobases. A 5.2-kilobase Pst-I fragment with F coordinates 38.9 to 44.1 has been cloned into a pSC101 vector to create the Pif+ plasmid pGS103. A series of Pif- deletion mutants and nonsense mutants were isolated from pGS103. Using minicells carrying pGS103 or its derivatives, we have identified a 70,000-dalton pif protein.

A partial restriction map of the proA-purE region of the Escherichia coli K12 chromosome

EcoRI restriction mapping data for fragments larger than 0.7 kb and contained in a 350-kb region of the Escherichia coli K-12 chromosome are presented. 75% of these fragments have been located relative to proA, B, argF, lac, proC, purE, and various insertion sequence elements normally present in this region. BglII and BamHI maps for the regions near argF and purE are also provided.

Gamma delta-mediated deletions of chromosomal segments on F-prime plasmids

Deleted derivatives of F lac+ proC+ tsx+/- purE+ plasmids ORF203 and F13 were isolated and physically characterized. Among 31 deletions, 24 were adjacent to the gamma delta element on F, four were associated with IS2 or IS3 elements normally present on F, and three displayed additional DNA rearrangements. With the genetic selection employed, the deletion endpoints in the chromosomal segment could fall anywhere within a 210 kb (5 min) region between proC and lac. The distribution of endpoints in this region was not random: the endpoints primarily occurred in an extended region near purE, and a 50 kb segment between tsx and purE was devoid of deletion endpoints. Deletion termini for mutants obtained from F13, which contains an additional 48 kb-segment interposed between gamma delta and the target region on ORF203, displayed a distribution similar to that seen for ORF203. Among simple deletions, there was no marked tendency for the chromosomal deletion endpoints to fall at IS1, IS3, or IS5 elements normally present in this chromosomal region. Point mutations and mutations caused by gamma delta or IS transposition into lac appeared in a small proportion of all plasmids studied.

Cloning and expression of the ilvB gene of Escherichia coli K-12

A plasmid containing the ilvB operon, which codes for acetohydroxy acid synthase I of Escherichia coli K-12, was isolated using a ligated mixture of DNA from plasmid pBR322 and FilvB4 treated with endonuclease SalI. A shortened derivative of this plasmid was isolated by cloning a 3.4 kb bacterial fragment into plasmid pKEN005 to yield plasmid pTCN12. The orientation of the ilvB operon relative to plasmid genes was determined by restriction enzyme mapping. Measurement of the level of the product of the ilvB gene, acetohydroxy acid synthase I, indicated that plasmid pTCN12 contained a functional ilvB promoter and control region. The DNA frm this plasmid was used as a probe to show that the rate of synthesis of ilvB mRNA was proportional to the levels of acetohydroxy acid synthase I.

Physical and genetic localization of ilv regulatory sites in lambda ilv bacteriophages

A set of nine lambda dilv phages were used to transduce bacterial recipients containing point mutations or deletions in the ilv genes located at 84 min on the Escherichia coli K-12 chromosome. This genetic analysis indicated that two phages carry the entire ilvGEDAC cluster; others carry the complete ilvC gene and, in addition, bacterial DNA that extends to a termination point between ilvA and ilvC, within ilvD, within ilvE, or within ilvG. DNA extracted from the lambda dilv phages was digested with EcoRI, HindIII, KpnI, PstI, SalI, and SmaI. The restriction maps revealed that these phages were generated after insertion at four distinct insertion sites downstream (clockwise) of ilvC. The physical relationships between the various phages were further examined by electron microscopic heteroduplex analysis. The physical maps of the phages thus generated were straightforward and in complete accord with the genetic data. No evidence for genetic rearrangements of ilv DNA in the phage was obtained, thus validating conclusions based on the use of these phages in previous and ongoing research projects. Bacterial cells with deletions of the ilv genes were made lysogenic with lambda dilv phage to examine the regulation of ilv genes present in the phage. The results confirm previous studies showing that one site for control by repression and derepression is upstream (counterclockwise) of ilvG. It was shown, in addition, that the activities of dihydroxy acid dehydrase and threonine deaminase were increased when the prototrophic lysogens were grown with 20 mM leucine. Since this increase was exhibited even when the ilvG-linked control region was not carried by the lambda dilv phage, additional control sites must be located within the ilvEDA region of the ilvGEDA transcription unit.

Cloning of a 1.1-kb fragment including srnB+ gene in the F plasmid and isolation of an srnB mutant

The srnB+ gene, promoting stable RNA degradation at 42 C in the presence of rifampin, was cloned by using pBR322 as a vector; it was located on a 1.1-kilobase (kb) Eco RI/Bam HI fragment between 1.4 and 2.5 kb of the F plasmid. The region between 93.3 and 4.0 kb of the F plasmid was physically mapped by using restriction endonucleases EcoRI, HindIII, BamHI, PstI, and SmaI, with reference to a standard HindIII site in IS3. An srnB1 mutant was isolated from a chimeric plasmid, pOY54, after treatment of its DNA with hydroxylamine. The srnB1 allele on the F fragment of the mutant plasmid was recessive to the wildtype allele. Thermal elevation of cell cultures to 39 C was high enough to promote RNA degradation in strain YS12 carrying plasmid pOY54.

Isolation and characterization of deletion mutants affected in early genes of bacteriophage phi 80

When Escherichia coli cells that had been irradiated with ultraviolet light were infected with bacteriophage phi 80, five major (pE, pB, pA, pC and pD) and two minor (pU and pV) proteins were found to be synthesized during early stages of infection. The genes coding for the five major proteins were mapped on the phi 80 chromosome using various deletion mutants which lacked the capacity to synthesize some or all the major proteins. The size and positions of all the deletions were determined by gel electrophoresis of EcoRi digests of phage DNA and by electron microscopy of heteroduplexes between DNAs of the deletion and wild-type phage. The five major proteins designated pE(25K), pB(40K), pA(45K), pC(34K) and pD(31K) were shown to be encoded in this order presumably by a single operon that was located at 60.2--67.4% on the phi 80 genome. These proteins were found to be involved in phage recombination. The absence of pE or pB resulted in a Red- phenotype and the absence of three proteins (pE, pB and pA) resulted in a Fec- phenotype. The exact positions of the genes for the minor proteins pU(29K) and pV(26K) have not been determined.

Genetical and structural analysis of a group of lambda ilv and lambda rho transducing phages

Eight lambda ilv C transducing phages generated from E. coli K12 secondary site lysogens have been analysed genetically and physically. Two of them carry, in addition, the rho gene and its promotor region, but not the cya gene. The ilv O 603 mutation has been located between ilv G and ilv E. Electrophoretic analysis of the proteins synthesized by these phages in a system of UV irradiated cells allowed us to assign molecular weights of 55000 and 66000 daltons to the ilv C and the ilv D gene products, respectively, and to show that an ilv G-encoded polypeptide of 60000 daltons is made from an ilv O- but not from an ilv O+ phage. The expression of the ilv G gene is discussed in the light of the recent finding of a promoter-attenuator region lying upstream to ilv G. Finally, we have found that one of the lambda ilv phages does not have the classical structure of a transducing phage.

Genetic and DNA sequence analysis of the kanamycin resistance transposon Tn903

The kanamycin resistance transposon Tn903 consists of a unique region of about 1000 base pairs bounded by a pair of 1050-base-pair inverted repeat sequences. Each repeat contains two Pvu II endonuclease cleavage sites separated by 520 base pairs. We have constructed derivatives of Tn903 in which this 520-base-pair fragment is deleted from one or both repeats. Those derivatives that lack both 520-base-pair fragments cannot transpose, whereas those that lack just one remain transposition proficient. One such transposable derivative, Tn903 delta I, has been selected for further study. We have determined the sequence of the intact inverted repeat. The 18 base pairs at each end are identical and inverted relative to one another, a structure characteristic of insertion sequences. Additional experiments indicate that a single inverted repeat from Tn903 can, in fact, transpose; we propose that this element be called IS903. To correlate the DNA sequence with genetic activities, we have created mutations by inserting a 10-base-pair DNA fragment at several sites within the intact repeat of Tn903 delta 1, and we have examined the effect of such insertions on transposability. The results suggest that IS903 encodes a 307-amino-acid polypeptide (a "transposase") that is absolutely required for transposition of IS903 or Tn903.

Regulation of ilvEDA expression occurs upstream of ilvG in Escherichia coli: additional evidence for an ilvGEDA operon

A low-copy-number plasmid was prepared that contained the entire ilv gene cluster of Escherichia coli. The introduction of an ilvO mutation allowed the ilvG gene of the plasmid to be expressed and imparted valine resistance to strains carrying it. Insertion of Tn10 into the ilvG gene of the plasmid resulted in a strong polar effect on ilv genes E, D, and A. Replacement of a region of ilv deoxyribonucleic acid between two KpnI sites on the high-copy-number plasmid carrying the entire ilv gene cluster with a KpnI fragment carrying an ilv-lac fusion but not extending into the ilv-specific control region resulted in a plasmid expressing the lacZ gene under ilv control when the fusion had been inserted in its normal orientation but not when it had been inserted in the opposite orientation. These experiments indicate that ilv-specific control over ilvE, ilvD, and ilvA expression is dependent on these genes being continguous with deoxyribonucleic acid that lies upstream of ilvG. The results also add further support to the concept of an ilvGEDA operon in E. coli.

Specificity in the formation of delta tra F-prime plasmids

Twenty-three independent delta tra F-prime plasmids from three different Escherichia coli K-12 sublines were isolated from Hfr strains whose points of origin coincided with the IS3 element alpha 3 beta 3 or alpha 4 beta 4 in the lac-purE region of the E. coli chromosome. Electrophoretic analysis of plasmid deoxyribonucleic acid digested with EcoRI and hybridization analysis of plasmid deoxyribonucleic acid digested with BglII revealed that at least 14 of these plasmids were formed by processes involving specific bacterial and F loci. Two of the specific bacterial loci involved in delta tra F-prime formation were located at approximately 3.3 and 11.7 min on the E. coli chromosomal map. Two of the delta tra F-prime plasmids contained bacterial deoxyribonucleic acid with circularization endpoints that mapped very near the termini of the IS2 element that is normally located between lac and proC.

Physical location of the ilvO determinant in Escherichia coli K-12 deoxyribonucleic acid

A plasmid carrying the 4,6-kilobase (kb) HindIII-derived fragment from an ilvO mutant derivative of lambda h80dilv imparted a valine-resistant phenotype on strains it carried. This fragment carries a small amount of the promoter-proximal end of ilvE, the ilvO determinant, and apparently the entire ilvG gene, which specifies the valine-insensitive acetohydroxy acid synthase. Comparable deoxyribonucleic acid (DNA) from the original lambda h80dilv did not carry the valine resistance marker. The valine-resistant phenotype was always correlated with the formation of the resistant enzymes. The ilvO determinant was shown to be carried within an approximately 600-based-pair region lying between the SalI and KpnI sites on the HindIII fragment and perhaps within the ilvG gene itself. Ribonucleic acid that hybridizes with the DNA corresponding to the ilvG gene is formed in wild-type K-12 cells. This fact, coupled with the fact that ilvG is transcribed from the same DNA strand as the ilvE, D, and A genes, led to the idea that transcription is normally initiated upstream from ilvG in both wild-type and ilvO strains. In wild-type strains either the formation or the translation of the transcript would be terminated with the ilvG gene, thus preventing expression of that gene.

Transposition of IS1-lambdaBIO-IS1 from a bacteriophage lambda derivative carrying the IS1-cat-IS1 transposon (Tn9)

Tn9 is a transposable element in which a gene (cat) determining chloramphenicol resistance is flanked by directly repeated sequences that are homologous to the insertion sequence IS1. We show here that infection of Escherichia coli K12 (under Rec-Red-Int- conditions) with a lambda bio transducing phage carrying Tn9 results in the formation of lambda bio transductants as frequently as cat transductants as frequently as cat transductants (about 1 per 10(6) to 10(7) infected cells). Most of the lambda bio transductants do not carry cat, just as most of the cat transductants do not carry lambda bio. In spite of the absence of cat, the lambda bio prophage can transpose a second time, from the E. coli chromosome to different sites on an F'gal plasmid. Analysis of the structure of the transposed lambda bio element, by restriction nuclease digestion and by electron microscopy, demonstrates that the integrated lambda bio prophage is flanked by directly repeated IS1 elements. We conclude that there is no genetic information for the ability to transpose encoded in the non-repeated portion of Tn9, i.e. that the directly repeated IS1 elements alone are responsible for Tn9 transposition.

Physical characterization of ilv-lac fusions

Electron microscopic heteroduplex analysis and comparative restriction digests have been used to characterize lambda p123(209), the complementary pair of phages used in the Casadaban technique of gene fusion. Derivatives of lambda 1(209) constructed to carry fusions of the lac genes to the control regions of the ilvC and ilvEDA operons were also analyzed. These physical maps have provided confirmation of the genetic models for these constructions and physical specifications important in interpreting the behavior of these ilv-lac fusions.

Specificity in formation of type II F' plasmids

Eight new F' plasmids derived from Hfr strains in which F is integrated at the chromosomal element alpha 3 beta 3 have been isolated and subjected to restriction enzyme, hybridization, and electron microscope heteroduplex analysis. Plasmids carrying extensive amounts of bacterial deoxyribonucleic acid were produced even though they were obtained by selection for transfer of lac, which is closely linked to F in the parental Hfr strains. Seven plasmids were type II Flac+ proC+ purE+ plasmids, and one was a type I Flac+ proC+ plasmid. Five of the Flac+ proC+ purE+ plasmids contain approximately 284 kilobases of bacterial deoxyribonucleic acid, which is identical for all five within the resolution of the restriction enzyme analysis. Theses results indicate that type II F' plasmids are the predominant tra+ F' type from this region of the Escherichia coli K-12 chromosome and that the recombination events leading to formation of these plasmids exhibit site specificity.

Genetic mapping of the chromosomal replication origin of Salmonella typhimurium

Two hundred strains of Escherichia coli harboring Filv+ plasmids which carry a segment of the Salmonella typhimurium chromosome were isolated independently. Among them, two strains were found to harbor F' plasmids that are able to replicate in Hfr cells of E. coli; i.e., they carry a site designated poh (permissive on Hfr) of the S. typhimurium chromosome. The poh site is presumably identical with the replication origin (oriC) of the bacterial chromosome. These two plasmids carry the dnaA-uncA-rbs-ilv-cya-metE region of the chromosome of S. typhimurium. Other F' plasmids which only carried the ilv-cya-metE region were unable to be maintained in Hfr cells. The poh site (= oriC) of S. typhimurium thus is located in the uhp-ilv region of the chromosome. The two plasmids carrying the poh site of S. typhimurium can suppress the temperature-sensitive character of an E. coli mutant that carries the temperature-sensitive dnaA46 allele, when the plasmids exist in the mutant cells. This suggests that the dnaA chromosome in place of the dnaA gene product of E. coli itself. The ability of the plasmids carrying the poh site of S. typhimurium to replicate in Hfr cells of E. coli suggests that the replication system of E. coli can recognize the Salmonella replication origin.

The structure of the phi 80d3 ilv+ Su+7 transducing phage and the origin of its Su+7 tRNA-gene containing fragment

The Bam HI, XhoI, and EcoRI sites of the transducing phage phi 80d3Su+7ilv+ are located. The 1.2 x 10(6) MD EcoRI fragment which, when cloned, contains tRNAAsp and expresses the mutant tRNATry gene, Su+7, and which also relaxes control of stable RNA synthesis is found immediately adjacent to the rrnC region. Its tRNA genes, tRNAAsp and tRNATry, are transcribed in the same direction as the ribosomal RNA genes, though no mature rRNA subsequences are on the fragment. This fragment also exists as such in another F-prime factor derived from the same Hfr host, and therefore presumably also in the Hfr chromosome itself. It is composed of about half ordinary chromosomal and half F DNA sequences, the latter from the gamma-delta region of F. The advantages of a novel mapping method used are discussed.

Investigations of the F conjugation gene traI:traI mutants and lambdatraI transducing phages

A series of traI point and deletion mutants of Flac, and a traM mutant, were characterised. Complementation tests with an amber Flac traI mutant confirmed their genotypes, and in addition all the traI mutants, but not the traM mutant, were complemented by pRS31 (PSC101 traDI) and EDlambda109 (lambdatraI). Judging from the efficiencies of plating of F-specific phages, none of the mutations affected pilus formation. The traI products of F and of the F-like plasmid R1 were interchangeable with each other but not with that of R100, while the traM product of F could not be replaced by those of R1 or of R100. Neither traI nor traM were needed for conjugal transfer of ColE1. Three lambda transducing phages carrying traI were isolated by in vivo or in vitro techniques, and characterised by genetic complementation tests, by analysis of the fragments produced by restriction endonucleases, and by measurement of heteroduplex molecules. The genetic structures together with the sizes and F coordinates, of the transfer regions carried by the phages were thereby determined. Comparison of the proteins synthesised in UV-irradiated cells by one of the lambdatraI phages with those made by a derivative carrying an amber traI mutation, allowed the traI product to be identified as a protein of molecular weight 174,000. In addition, the molecular weights of the traD (84,000), traS (18,000), and traT (25,000) products made by the lambdatraSTD1 phage EDlambda107 were measured. The possible roles of the traI and traM products in conjugation are discussed.

Deletion mapping of the ilvGOEDAC genes of Escherichia coli K-12

A set of lambdadilv phage has been examined that carry overlapping segments of isoleucine-valine structural and regulatory genes derived from the ilv cluster at 83 min on the Escherichia coli K-12 chromosome. The ilv genes present in these phage, and their order, have been determined by transduction of auxotrophs, escape synthesis, and deletion mapping. The order of ilv genes in the phage, and hence the order in the host chromosome, was found to be ilvG-ilvO-ilvEDA-ilvC. Lysogens containing lambdadilv phage were constructed for dominance analysis of regulatory mutations in the ilvO and ilvA genes. The ilvO671 allele is cis-dominant to ilvO+, while the ilvA538 allele is trans-recessive to ilvA+. Thus, the ilvO gene, that is identified by cis-dominant regulatory mutations that result in increased ilvG and ilvEDA expression, is situated between and may be contiguous with ilvG and ilvEDA.

Homology of the gene coding for outer membrane lipoprotein within various Gram-negative bacteria

The mRNA for a major outer membrane lipoprotein from Escherichia coli was found to hybridize specifically with one of the EcoRI and one of the HindIII restriction endonuclease-generated fragments of total DNA from nine bacteria in the family Enterobacteriaceae: E. coli, Shigella dysenteriae, Salmonella typhimurium, Citrobacter freundii, Klebsiella aerogenes, Enterobacter aerogenes, Edwardsiella tarda, Serratia marcescens, and Erwinia amylovora. However, among the Enterobacteriaceae, DNA from two species of Proteus (P. mirabilis and P. morganii) did not contain any restriction endonuclease fragments that hybridized with the E. coli lipoprotein mRNA. Furthermore, no hybrid bands were detected in four other gram-negative bacteria outside the family Enterobacteriaceae: Pseudomonas aeruginosa, Acinetobacter sp. HO1-N, Caulobacter crescentus, and Myxococcus xanthus. Envelope fractions from all bacteria in the family Enterobacteriaceae tested above cross-reacted with antiserum against the purified E. coli free-form lipoprotein in the Ouchterlony immunodiffusion test. Both species of Proteus, however, gave considerably weaker precipitation lines, in comparison with the intense lines produced by the other members of the family. All of the above four bacteria outside the family Enterobacteriaceae did not cross-react with anti-E. coli lipoprotein serum. From these results, the rate of evolutionary changes in the lipoprotein gene seems to be closely related to that observed for various soluble enzymes of the Enterobacteriaceae.

Suppressors of a genetic regulatory mutation affecting isoleucine-valine biosynthesis in Escherichia coli K-12

Escherichia coli K-12 mutant PS187 carries a mutation, ilvA538, in the structural gene for the biosynthetic L-threonine deaminase that leads to a leucine-sensitive growth phenotype, an isoleucine- and leucine-hypersensitive L-threonine deaminase, and pleiotropic effects resulting in abnormally low and invariant expression of some of the isoleucine-valine biosynthetic enzymes. Fifty-eight derivatives of strain PS187 were isolated as resistant to growth inhibition by leucine, by valine, or by valine plus glycly-valine and were biochemically, genetically, and physiologically characterized. All of these derivatives produced the feedback-hypersensitive L-threonine deaminase, and thus presumably possess the ilvA538 allele of the parent strain. Elevated synthesis of L-threonine deaminase was observed in 41 of the 58 isolates. Among 18 strains analyzed genetically, only those with mutations linked to the ilv gene clusters at 83 min produced elevated levels of L-threonine deaminase. One of the strains, MSR91, isolated as resistant to valine plus glycyl-valine, was chosen for more detailed study. The locus in strain MSR91 conferring resistance was located in four factor crosses between ilvE and rbs, and is in or near the ilvO gene postulated to be a site controlling the expression of the ilvEDA genes. Synthesis of the ilvEDA gene products in strain MSR91 is constitutive and derepressed approximately 200-fold relative to the parent strain, indicating that the genetic regulatory effects of the ilvA538 allele have been suppressed. Strain MSR91 should be suitable for use in purification of the ilvA538 gene product, since enzyme synthesis is fully derepressed and the suppressor mutation is clearly not located within the ilvA gene.