Identification of a complex operator for galP1, the glucose-sensitive, galactose-dependent promoter of the Streptomyces galactose operon

Comparison of actinomycin peptide synthetase formation in Streptomyces chrysomallus and Streptomyces antibioticus

Actinomycin peptide synthetase genes constitute two oppositely oriented transcriptional units, acmADR, and acmBC, separated by a non-coding intergenic region. Gene constructs of the intergenic region together with its adjoining gene acmA or acmB from the actinomycin biosynthetic gene cluster of Streptomyces chrysomallus were transferred into Streptomyces lividans TK64. Each construct expressed the respective synthetase indicating divergent promoters. Primer extension revealed for both directions -10 and -35 boxes similar to σ⁷⁰ -dependent promoters from Streptomyces and E. coli. No conspicuous regulatory sequences were detected. Accordingly, S. chrysomallus-grown in glucose-containing medium-produced the peptide synthetases AcmA and AcmB/C as well as actinomycin during logarithmic growth phase. Alignments with the corresponding intergenic region of the actinomycin biosynthetic gene cluster in Streptomyces antibioticus identified analogous -10 and -35 boxes of σ⁷⁰ consensus sequence. However, in S. antibioticus-cultivated in the same conditions-AcmA and AcmB/C were at maximum activity in late log phase and actinomycin formation peaked in stationary phase. The different patterns of formation of actinomycin and its peptide synthetases encoded by the highly homologous actinomycin biosynthetic gene clusters in S. chrysomallus and S. antibioticus suggest strain-specific control of biosynthesis in agreement with absence of pathway-specific regulatory genes.

Expression, purification and functional characterization of AmiA of acetamidase operon of Mycobacterium smegmatis

Regulation of gene expression is one of the mechanisms of virulence in pathogenic organisms. In this context, we would like to understand the gene regulation of acetamidase enzyme of Mycobacterium smegmatis, which is the first reported inducible enzyme in mycobacteria. The acetamidase is highly inducible and the expression of this enzyme is increased 100-fold when the substrate acetamide is added. The acetamidase structural gene (amiE) is found immediately downstream of three predicted open reading frames (ORFs). Three of these genes along with a divergently expressed ORF are predicted to form an operon and involved in the regulation of acetamidase enzyme. Here we report expression, purification and functional characterization of AmiA which is one of these predicted ORFs. Electrophoretic mobility shift assays showed that AmiA binds to the region between the amiA and amiD near the predicted promoter (P2). Over-expression of AmiA significantly lowered the expression of acetamidase compared to the wild type as demonstrated by qRT-PCR and SDS-PAGE. We conclude that AmiA binds near P2 promoter and acts as a repressor in the regulation of acetamidase operon. The described work is a further step forward toward broadening the knowledge on understanding of the complex gene regulatory mechanism of Mycobacterium sp.

How phosphotransferase system-related protein phosphorylation regulates carbohydrate metabolism in bacteria

The phosphoenolpyruvate(PEP):carbohydrate phosphotransferase system (PTS) is found only in bacteria, where it catalyzes the transport and phosphorylation of numerous monosaccharides, disaccharides, amino sugars, polyols, and other sugar derivatives. To carry out its catalytic function in sugar transport and phosphorylation, the PTS uses PEP as an energy source and phosphoryl donor. The phosphoryl group of PEP is usually transferred via four distinct proteins (domains) to the transported sugar bound to the respective membrane component(s) (EIIC and EIID) of the PTS. The organization of the PTS as a four-step phosphoryl transfer system, in which all P derivatives exhibit similar energy (phosphorylation occurs at histidyl or cysteyl residues), is surprising, as a single protein (or domain) coupling energy transfer and sugar phosphorylation would be sufficient for PTS function. A possible explanation for the complexity of the PTS was provided by the discovery that the PTS also carries out numerous regulatory functions. Depending on their phosphorylation state, the four proteins (domains) forming the PTS phosphorylation cascade (EI, HPr, EIIA, and EIIB) can phosphorylate or interact with numerous non-PTS proteins and thereby regulate their activity. In addition, in certain bacteria, one of the PTS components (HPr) is phosphorylated by ATP at a seryl residue, which increases the complexity of PTS-mediated regulation. In this review, we try to summarize the known protein phosphorylation-related regulatory functions of the PTS. As we shall see, the PTS regulation network not only controls carbohydrate uptake and metabolism but also interferes with the utilization of nitrogen and phosphorus and the virulence of certain pathogens.

Sugar uptake and sensitivity to carbon catabolite regulation in Streptomyces peucetius var. caesius

Streptomyces peucetius var. caesius produces a family of secondary metabolites called anthracyclines. Production of these compounds is negatively affected in the presence of glucose, galactose, and lactose, but the greatest effect is observed under conditions of excess glucose. Other carbon sources, such as arabinose or glutamate, show either no effect or stimulate production. Among the carbon sources that negatively affect anthracycline production, glucose is consumed in greater concentrations. We determined glucose and galactose transport in S. peucetius var. caesius and in a mutant of this strain whose anthracycline production is insensitive to carbon catabolite repression (CCR). In the original strain, incorporation of glucose and galactose was stimulated when the microorganism was grown in media containing these sugars, although we also observed basal galactose incorporation. Both the induced and the basal incorporation of galactose were suppressed when the microorganism was grown in the presence of glucose. Furthermore, adding glucose directly during the transport assay also inhibited galactose incorporation. In the mutant strain, we observed a reduction in both glucose (48%) and galactose (81%) incorporation compared to the original. Galactose transport in this mutant showed reduced sensitivity to the negative effect of glucose; however, it was still sensitive to inhibition. The deficient transport of these sugars, as well as CCR sensitivity to glucose in this mutant was corrected when the mutant was transformed with the SCO2127 region of the Streptomyces coelicolor genome. Our results support a role for glucose as the most easily utilized carbon source capable of exerting the greatest repression on anthracycline biosynthesis. In consequence, glucose also prevented the repressive effect of galactose by suppressing its incorporation. This suggests the participation of an integral regulatory system, which is initiated by an increase in incorporation of repressive sugars and their metabolism as a prerequisite for establishing the phenomenon of CCR in S. peucetius var. caesius.

Glucose kinase alone cannot be responsible for carbon source regulation in Streptomyces peucetius var. caesius

Using an antibiotic enrichment procedure, eight mutants of Streptomyces peucetius var. caesius were isolated for their sensitivity to the glucose analogue 2-deoxyglucose (DOG), from a DOG-resistant strain (Dog(R)). These mutants (Dog(S)) and their parent strain were examined for growth sensitivity to DOG, glucose kinase (Glk) activity, glucose uptake, and sensitivity to repression by glucose and other catabolites derived from it. No correlation was found between Glk levels or glucose uptake and carbon catabolite repression (CCR) in these strains. However, the ratio of glucose uptake to Glk activity, and thus the flux through glycolysis, seemed responsible for this effect. Among several products of glucose catabolism tested, fructose-1,6-bis-phosphate and phosphoenolpyruvate showed significant repression of anthracycline formation. These compounds also reduced anthracycline formation in a Dog(R) mutant insensitive to glucose repression. Our data suggest that Glk alone is not sufficient to elicit CCR in this microorganism, and gives the first physiological evidence supporting the hypothesis that some products of glucose catabolism are involved in CCR in Streptomyces.

In silico and transcriptional analysis of carbohydrate uptake systems of Streptomyces coelicolor A3(2)

Streptomyces coelicolor is the prototype for the investigation of antibiotic-producing and differentiating actinomycetes. As soil bacteria, streptomycetes can metabolize a wide variety of carbon sources and are hence vested with various specific permeases. Their activity and regulation substantially determine the nutritional state of the cell and, therefore, influence morphogenesis and antibiotic production. We have surveyed the genome of S. coelicolor A3(2) to provide a thorough description of the carbohydrate uptake systems. Among 81 ATP-binding cassette (ABC) permeases that are present in the genome, we found 45 to encode a putative solute binding protein, an essential feature for carbohydrate permease function. Similarity analysis allowed the prediction of putative ABC systems for transport of cellobiose and cellotriose, alpha-glucosides, lactose, maltose, maltodextrins, ribose, sugar alcohols, xylose, and beta-xylosides. A novel putative bifunctional protein composed of a substrate binding and a membrane-spanning moiety is likely to account for ribose or ribonucleoside uptake. Glucose may be incorporated by a proton-driven symporter of the major facilitator superfamily while a putative sodium-dependent permease of the solute-sodium symporter family may mediate uptake of galactose and a facilitator protein of the major intrinsic protein family may internalize glycerol. Of the predicted gene clusters, reverse transcriptase PCRs showed active gene expression in 8 of 11 systems. Together with the previously surveyed permeases of the phosphotransferase system that accounts for the uptake of fructose and N-acetylglucosamine, the genome of S. coelicolor encodes at least 53 potential carbohydrate uptake systems.

A common precursor for the three subunits ofL-glutamate oxidase encoded bygoxgene fromStreptomyces platensisNTU3304

A segment of DNA containing the L-glutamate oxidase (gox) gene from Streptomyces platensis NTU3304 was cloned. The entire nucleotide sequence of the protein-coding portion consisting of 2130 bp (710 codons, including AUG and UGA) of the cloned DNA fragment was determined. The gox gene contained only one open reading frame (ORF) which coded for a 78-kDa polypeptide, the precursor of active extracellular Gox. Mature Gox is composed of three subunits, designated as α, β, and γ, with molecular masses of 39, 19, and 16 kDa, respectively. Analyses of the N-terminal amino acid sequences of the subunits revealed that the order of subunits in the precursor polypeptide encoded by the ORF, from N-terminus to C-terminus, is α–γ–β. The presence of the flavin adenine dinucleotide (FAD)-binding motif place Gox as a member of the flavoenzyme family. Furthermore, a negative effect of glucose on the biosynthesis of Gox was observed when it was used as carbon source.Key words: L-glutamate oxidase, gox gene, signal peptide, DNA sequence, flavoenzyme, pIJ702 vector.

Primary metabolism and its control in streptomycetes: a most unusual group of bacteria

Streptomycetes are Gram-positive bacteria with a unique capacity for the production of a multitude of varied and complex secondary metabolites. They also have a complex life cycle including differentiation into at least three distinct cell types. Whilst much attention has been paid to the pathways and regulation of secondary metabolism, less has been paid to the pathways and the regulation of primary metabolism, which supplies the precursors. With the imminent completion of the total genome sequence of Streptomyces coelicolor A3(2), we need to understand the pathways of primary metabolism if we are to understand the role of newly discovered genes. This review is written as a contribution to supplying these wants. Streptomycetes inhabit soil, which, because of the high numbers of microbial competitors, is an oligotrophic environment. Soil nutrient levels reflect the fact that plant-derived material is the main nutrient input; i.e. it is carbon-rich and nitrogen- and phosphate-poor. Control of streptomycete primary metabolism reflects the nutrient availability. The variety and multiplicity of carbohydrate catabolic pathways reflects the variety and multiplicity of carbohydrates in the soil. This multiplicity of pathways has led to investment by streptomycetes in pathway-specific and global regulatory networks such as glucose repression. The mechanism of glucose repression is clearly different from that in other bacteria. Streptomycetes feed by secreting complexes of extracellular enzymes that break down plant cell walls to release nutrients. The induction of these enzyme complexes is often coordinated by inducers that bear no structural relation to the substrate or product of any particular enzyme in the complex; e.g. a product of xylan breakdown may induce cellulase production. Control of amino acid catabolism reflects the relative absence of nitrogen catabolites in soil. The cognate amino acid induces about half of the catabolic pathways and half are constitutive. There are reduced instances of global carbon and nitrogen catabolite control of amino acid catabolism, which again presumably reflects the relative rarity of the catabolites. There are few examples of feedback repression of amino acid biosynthesis. Again this is taken as a reflection of the oligotrophic nature of the streptomycete ecological niche. As amino acids are not present in the environment, streptomycetes have rarely invested in feedback repression. Exceptions to this generalization are the arginine and branched-chain amino acid pathways and some parts of the aromatic amino acid pathways which have regulatory systems similar to Escherichia coli and Bacillus subtilis and other copiotrophic bacteria.

Glutamate and cyclic AMP regulate the expression of galactokinase in Mycobacterium smegmatis

It was found that Mycobacterium smegmatis is unable to utilize galactose as the sole carbon source because the sugar alone cannot induce galactokinase. However, galactokinase was induced by glutamate alone, and was further stimulated by galactose. Rifampicin completely inhibited the glutamate-mediated expression of galK in both the absence and presence of galactose. Extracellular cAMP stimulated the expression of the enzyme only in the presence of glutamate plus galactose. The galK gene from M. smegmatis, including its upstream promoter region, was cloned in a plasmid in Escherichia coli. The expression of kinase from these clones in E. coli was dependent on cAMP and its receptor protein (CRP). The expression of UDP-galactose 4-epimerase was constitutive. This and other evidence suggests that the galK gene is not linked to galT and galE in the mycobacterial genome. In a glutamate-independent galactose-utilizing mutant (gin-1 mutant) of M. smegmatis, galK was expressed in the absence of both galactose and glutamate, while in the presence of galactose this expression was increased twofold in the absence of glutamate and fourfold in its presence. Extracellularly added cAMP reduced the expression of the enzyme in the presence of galactose plus glutamate nearly to the basal level. It is proposed that in M. smegmatis the galK gene is expressed from two different promoters; the expression from one promoter is dependent on glutamate but not on galactose and cAMP, while that from the other requires all three components. The role of galactose is possibly to derepress the latter promoter.

The bldB gene encodes a small protein required for morphogenesis, antibiotic production, and catabolite control in Streptomyces coelicolor

Mutants blocked at the earliest stage of morphological development in Streptomyces species are called bld mutants. These mutants are pleiotropically defective in the initiation of development, the ability to produce antibiotics, the ability to regulate carbon utilization, and the ability to send and/or respond to extracellular signals. Here we report the identification and partial characterization of a 99-amino-acid open reading frame (ORF99) that is capable of restoring morphogenesis, antibiotic production, and catabolite control to all of the bldB mutants. Of the existing bld mutants, bldB is of special interest because the phenotype of this mutant is the most pleiotropic. DNA sequence analysis of ORF99 from each of the existing bldB mutants identified base changes either within the coding region of the predicted protein or in the regulatory region of the gene. Primer extension analysis identified an apparent transcription start site. A promoter fusion to the xylE reporter gene showed that expression of bldB is apparently temporally regulated and that the bldB gene product is involved in the regulation of its own expression.

Direct repeat sequences in the Streptomyces chitinase-63 promoter direct both glucose repression and chitin induction

The chi63 promoter directs glucose-sensitive, chitin-dependent transcription of a gene involved in the utilization of chitin as carbon source. Analysis of 5' and 3' deletions of the promoter region revealed that a 350-bp segment is sufficient for wild-type levels of expression and regulation. The analysis of single base changes throughout the promoter region, introduced by random and site-directed mutagenesis, identified several sequences to be important for activity and regulation. Single base changes at -10, -12, -32, -33, -35, and -37 upstream of the transcription start site resulted in loss of activity from the promoter, suggesting that bases in these positions are important for RNA polymerase interaction. The sequences centered around -10 (TATTCT) and -35 (TTGACC) in this promoter are, in fact, prototypical of eubacterial promoters. Overlapping the RNA polymerase binding site is a perfect 12-bp direct repeat sequence. Some base changes within this direct repeat resulted in constitutive expression, suggesting that this sequence is an operator for negative regulation. Other base changes resulted in loss of glucose repression while retaining the requirement for chitin induction, suggesting that this sequence is also involved in glucose repression. The fact that cis-acting mutations resulted in glucose resistance but not inducer independence rules out the possibility that glucose repression acts exclusively by inducer exclusion. The fact that mutations that affect glucose repression and chitin induction fall within the same direct repeat sequence module suggests that the direct repeat sequence facilitates both chitin induction and glucose repression.

Genetic and biochemical characterization of a new extracellular lipase from Streptomyces cinnamomeus

Streptomyces cinnamomeus Tü89 secretes a 30-kDa esterase and a 50-kDa lipase. The lipase-encoding gene, lipA, was cloned from genomic DNA into Streptomyces lividans TK23 with plasmid vector pIJ702. Two lipase-positive clones were identified; each recombinant plasmid had a 5.2-kb MboI insert that contained the complete lipA gene. The two plasmids differed in the orientation of the insert and the degree of lipolytic activity produced. The lipA gene was sequenced; lipA encodes a proprotein of 275 amino acids (29,213 Da) with a pI of 5.35. The LipA signal peptide is 30 amino acids long, and the mature lipase sequence is 245 amino acids long (26.2 kDa) and contains six cysteine residues. The conserved catalytic serine residue of LipA is in position 125. Sequence similarity of the mature lipases (29% identity, 60% similarity) was observed mainly in the N-terminal 104 amino acids with the group II Pseudomonas lipases; no similarity to the two Streptomyces lipase sequences was found. lipA was also expressed in Escherichia coli under the control of lacZ promoter. In the presence of the inducer isopropyl-beta-D-thiogalactopyranoside (IPTG), growth of the E. coli clone was severely affected, and the cells lysed in liquid medium. Lipase activity in the E. coli clone was found mainly in the pellet fraction. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, three additional protein bands of 50, 29, and 27 kDa were visible. The 27-kDa protein showed lipolytic activity and represents the mature lipase; the 29- and 50-kDa forms showed no activity and very probably represent the unprocessed form and a dimeric misfolded form, respectively. For higher expression of lipA in S. lividans, the gene was cloned next to the strong aphII promoter. In contrast to the lipA-expressing E. coli clone, S. cinnamomeus and the corresponding S. lividans clone secreted only an active protein of 50 kDa. The lipase showed highest activity with C6 and C18 triglycerides; no activity was observed with phospholipids, Tween 20, or p-nitrophenylesters. Upstream of lipA and in the same orientation, an open reading frame, orfA, is found whose deduced protein sequence (519 amino acids) shows similarity to various membrane-localized transporters. Downstream of lipA and in the opposite orientation, an open reading frame, orfB (encoding a 199-amino-acid protein) is found, which shows no conspicuous sequence similarity to known proteins, other than an NAD and flavin adenine dinucleotide binding-site sequence.

The Streptomyces galP1 promoter has a novel RNA polymerase recognition sequence and is transcribed by a new form of RNA polymerase in vitro

We report the identification of DNA sequences that determine the activity of the Streptomyces galP1 promoter and a new form of RNA polymerase holoenzyme that recognizes these sequences in vitro. Base substitutions were introduced throughout the galP1 promoter region, and bases at positions -34, -36, and -11 with respect to the transcription start site were shown to be required for promoter function. These bases correspond in their positions to regions known to be important for RNA polymerase binding in several classes of eubacterial promoters, but the sequences themselves are not similar to those previously described. The -35 region of the galP1 promoter consists of six G residues, and base changes in this G hexamer had a dramatic effect on promoter activity. By using galP1-containing DNA template, a new RNA polymerase activity was purified from Streptomyces. Holoenzyme reconstitution experiments identified a new sigma factor that directs galP1 transcription in vitro. DNase I protection experiments identified a binding site for this new holoenzyme immediately upstream of the galP1 transcription start site.

Differential expression of two sporulation specific sigma factors of Streptomyces aureofaciens correlates with the developmental stage

In previous experiments, the Streptomyces aureofaciens (Sa) rpoZ, and sigF genes have been shown to encode putative sigma factors essential in differentiation. In an attempt to investigate expression of these genes during differentiation, we have performed S1 nuclease mapping using RNA prepared from Sa in various developmental stages. Two putative promoters were identified upstream of the rpoZ coding region. The promoters significantly differed in their strength, and were active in distinct developmental stages; the weaker, rpoZ-P1, was active only in substrate mycelium, and the stronger, rpoZ-P2, was induced at the beginning of aerial mycelium formation. Transcriptional analysis of sigF revealed two apparent transcription start points, both being detectable only in the late phase of aerial mycelium formation. No sigF transcription was detected in an rpoZ-disrupted Sa strain. Promoter-bearing DNA fragments from rpoZ and sigF were inserted into several promoter-probe vectors, to give expression patterns consistent with the results of direct RNA analysis. The results implicate temporally different expression of rpoZ and sigF during the differentiation of Sa, and direct or indirect dependence of sigF expression on the rpoZ-encoded putative sigma factor, thus indicating a cascade of sigma factors in Streptomyces development.

Sequence and transcript analysis of a novel Methanosarcina barkeri methyltransferase II homolog and its associated corrinoid protein homologous to methionine synthase

The sequence and transcript of the genes encoding a recently discovered coenzyme M methylase in Methanosarcina barkeri were analyzed. This 480-kDa protein is composed of two subunits in equimolar concentrations which bind one corrinoid cofactor per alphabeta dimer. The gene for the alphabeta polypeptide, mtsA, is upstream of that encoding the beta polypeptide, mtsB. The two genes are contiguous and overlap by several nucleotides. A 1.9-kb mRNA species which reacted with probes specific for either mtsA or mtsB was detected. Three possible methanogen consensus BoxA sequences as well as two sets of direct repeats were found upstream of mtsA. The 5' end of the mts transcript was 19 nucleotides upstream of the translational start site of mtsA and was positioned 25 bp from the center of the proximal BoxA sequence. The transcript was most abundant in cells grown to the late log phase on acetate but barely detectable in cells grown on methanol or trimethylamine. The amino acid sequence of MtsB was homologous to the cobalamin-binding fragment of methionine synthase from Escherichia coli and possessed the signature residues involved in binding the corrinoid, including a histidyl residue which ligates cobalt. The sequence of MtsA is homologous to the "A" and "M" isozymes of methylcobamide:coenzyme M methyltransferases (methyltransferase II), indicating that the alpha polypeptide is a new member of the methyltransferase II family of coenzyme M methylases. All three methyltransferase II homolog sequences could be aligned with the sequences of uroporphyrinogen decarboxylase from various sources. The implications of these homologies for the mechanism of corrinoid binding by proteins involved in methylotrophic methanogenesis are discussed.

A sequence-specific DNA-binding protein interacts with the xlnC upstream region of Streptomyces sp. strain EC3

The alignment of the promoter region of several Streptomyces xylanases shows three conserved sequences which could be involved in gene regulation. By electromobility shift assays these specific sequences, present only in Streptomyces xylanolytic strains, were identified as protein-binding sites. The sequence required for efficient recognition by the retarding protein appeared to be a 4-bp inverted repeat: 5'-CTTT-Nx-AAAG-3'. The DNA-protein affinity was influenced by the culture conditions.

Nucleotide sequence, transcriptional analysis, and glucose regulation of the phenoxazinone synthase gene (phsA) from Streptomyces antibioticus

The nucleotide sequence of a 2.3-kb SphI fragment containing the structural gene (phsA) for phenoxazinone synthase (PHS) of Streptomyces antibioticus was determined. The sequence was found to contain an open reading frame (ORF) with a G+C content of 71.5% oriented in the direction of transcription that was confirmed by primer extension. The ORF encodes a protein with an M(r) of 70,223 consisting of 642 amino acids and is preceded by a potential ribosome-binding site. The codon usage pattern is in agreement with the general pattern for streptomycete genes, with a 92.5 mol% G+C content in the third position. The N-terminal sequence of the mature PHS subunit corresponds exactly to that predicted from the nucleotide sequence. Neither ATG nor GTG initiator codons were identified for the protein. However, a TTG codon was located near the amino terminus of the mature protein and is a good candidate for the initiator codon. The transcriptional start point of phsA was located 36 bp upstream of the start codon by primer extension. The -10 region of the putative promoter showed some similarity to the consensus sequence for the major class of prokaryotic promoters, but the -35 region was less similar. Comparison of the primary amino acid sequence of PHS of S. antibioticus with other amino acid sequences indicated that PHS is a blue copper protein with copper binding domains in the N-terminal and C-terminal regions of the polypeptide chain. A BsrBI fragment containing the promoter region of phsA and a portion of the ORF was shown to promote xylE expression when cloned in the streptomycete promoter probe vector pIJ2843. This phsA promoter-dependent xylE expression could be repressed by glucose in S. antibioticus when the organism was grown on glucose or galactose plus glucose. Thus, the cloned promoter region appears to contain the sequences responsible for catabolite repression of PHS production.

Regulation of expression of the valine (branched-chain amino acid) dehydrogenase-encoding gene from Streptomyces coelicolor

Expression of the Streptomyces coelicolor (Sc) valine (branched-chain amino acid) dehydrogenase-encoding gene (vdh) is regulated by valine, glucose and NH+4 at the transcriptional level. The results of assays for the level of accumulated vdh mRNA in the Sc J802 strain by primer extension experiments and for the level of catechol dioxygenase (XylE) activity in Sc J802 (vdh::xylE) transformants show that transcription of the vdh gene is induced approx. 2.5-fold by valine, as compared to asparagine as the sole nitrogen source in the presence of glucose as carbon source. Valine induction is repressed by glucose, as compared to glycerol as the carbon source, and by NH+4. Glucose catabolite repression is relieved in Sc M480, a glucose kinase (glkA) deletion mutant. This suggests that glucose repression of vdh and carbohydrate metabolism are due to the same mechanism in Sc, which involves glucose kinase.

ccrA1: a mutation in Streptomyces coelicolor that affects the control of catabolite repression

The regulation of carbon utilization is of central importance in the gene expression pathways for both morphological development and antibiotic production in Streptomyces species. We report the identification and characterization of a mutation in Streptomyces coelicolor, ccrA1, that affects the expression of several catabolite-controlled promoters. ccrA1 mutants are altered in expression of galP1, the glucose-sensitive, galactose-dependent promoter of the galactose utilization operon; in expression of the glycerol utilization operon, which is glucose sensitive and glycerol dependent; and in expression of chi63, the glucose-sensitive chitin-dependent promoter of a gene involved in chitin utilization. ccrA1 has no effect on the expression of galP2, a promoter that directs constitutive transcription of the galE and galK genes. ccrA1 maps to a region of the S. coelicolor genome which distinguishes it from other mutations known to be involved in catabolite control. We suggest that ccrA1 identifies a gene whose product may be involved in the general regulation of carbon catabolite repression in this complex bacterium.

Advances in heterologous gene expression by Streptomyces

During the past year, noteworthy advances have been made in our ability to express foreign genes in Streptomyces. These advances were due, in part, to a detailed examination of the critical parameters that limit expression by Streptomyces of soluble forms of the human T-cell receptor CD4. Significant progress has also been made in our understanding of transcriptional regulation and protease gene expression. Application of this knowledge to expression vector design and the construction of alternative expression hosts should improve our ability to easily and routinely express foreign genes in Streptomyces.

Substrate induction and catabolite repression of the Streptomyces coelicolor glycerol operon are mediated through the GylR protein

The pathway for glycerol catabolism in Streptomyces coelicolor is determined by the gylCABX operon, which is transcribed from two closely spaced glycerol-inducible, glucose-repressible promoters. Glucose (or catabolite) repression of gyl is known to be exerted by a general catabolite repression system in which the soluble glucose kinase plays a central role. The gylR gene is contained in a separate glycerol-inducible, weakly glucose-repressible transcription unit immediately upstream from the gyl operon. The role of gylR in the regulation of gyl transcription was assessed by introducing specific null mutations into the chromosomal gylR gene. Direct quantification of gyl transcripts from the gylR null mutants grown on different carbon sources demonstrated that GylR is the repressor of the gylCABX operon and also revealed that GylR functions as a negative autoregulator. Moreover, the transcriptional analysis revealed that the gylR null mutants were relieved of glucose repression of both gylCABX and gylR. We conclude that both substrate induction and catabolite repression of gyl are mediated through the GylR protein. This is the first direct evidence that catabolite repression in Streptomyces is not exerted at the transcriptional level by a general 'catabolite repressor protein'. Models for catabolite repression are discussed.