The active site regions of lacZ and ebg beta-galactosidases are homologous

Purification and molecular characterization of cold-active β-galactosidase from Arthrobacter psychrolactophilus strain F2

In this study, we purified and molecularly characterized a cold-active beta-galactosidase from Arthrobacter psychrolactophilus strain F2. The purified beta-galactosidase from strain F2 exhibited high activity at 0 degrees C, and its optimum temperature and pH were 10 degrees C and 8.0, respectively. It was possible to inactivate the beta-galactosidase rapidly at 45 degrees C in 5 min. The enzyme was able to hydrolyze lactose as a substrate, as well as o-nitrophenyl-beta-D-galactopyranoside (ONPG), the Km values with ONPG and lactose being calculated to be 2.8 mM and 50 mM, respectively, at 10 degrees C. Moreover, the bglA gene encoding the beta-galactosidase of strain F2 was cloned and analyzed. The bglA gene consists of a 3,084-bp open reading frame corresponding to a protein of 1,028 amino acid residues. BglAp, the gene product derived from bglA, had several conserved regions for glycosyl hydrolase family 2, e.g., the glycosyl hydrolase 2 (GH2) sugar binding domain, GH2 acid-base catalyst, GH2 triosephosphate isomerase barrel domain, GH2 signature 1, and several other GH2 conserved regions. From these facts, we conclude that the beta-galactosidase from A. psychrolactophilus strain F2, which is a new member of glycosyl hydrolase family 2, is a cold-active enzyme that is extremely heat labile and could have advantageous applications in the food industry.

Approaches to labeling and identification of active site residues in glycosidases

Glycosidases play a key role in a number of biological processes and, as such, are of considerable clinical and biotechnological importance. Knowledge of the identifies of catalytically important active site residues is essential for understanding the catalytic mechanism, for enzyme classification, and for targeted bioengineering of glycosidases with altered characteristics. Here we review and discuss traditional strategies and novel approaches based on tandem mass spectrometry for the identification of the key active site residues in glycosidases.

Identification and sequencing of the Thermotoga maritima lacZ gene, part of a divergently transcribed operon

The lacZ gene encoding a beta-galactosidase (beta Gal) from the hyperthermophile Thermotoga maritima was cloned on an 11-kb fragment by complementation of an Escherichia coli lacZ deletion stain. The nucleotide sequence of the structural gene and two other ORFs found within a 6317-bp region were determined. The deduced amino acid (aa) sequence of the Tt. maritima beta Gal predicts a 1037-aa polypeptide with a calculated M(r) of 122,312. The translated sequence is 30% similar to nine other beta Gal sequences from bacteria and one yeast. Alignment of the Tt. maritima beta Gal with these other sequences reveals that the residues responsible for Mg2+ binding, catalysis and substrate recognition are conserved in the thermophilic enzyme. Sequence analysis also revealed the presence of a divergently transcribed operon containing at least two other genes 5' to lacZ. These ORFs encode proteins homologous to a second family of beta Gal found in Bacillus species and to an ATP-dependent family of bacterial oligopeptide transport proteins.

LacZ transgenic mouse models: their application in genetic toxicology

Gene mutations have been implicated in the etiology of cancer, developmental anomalies, genetic disease and aging. Many different methods for mutation detection have been developed and applied to obtain a more fundamental insight in the chain of molecular events that ultimately lead to mutations. Most of these methods, however, can only be applied to cultured cells and therefore do not allow comparative analysis of mutations in various organs and tissues in an intact organism. The main difficulty in studying mutagenesis in chromosomal DNA is to identify and isolate mutated genes with a high efficiency. Here we describe the development and application of LacZ transgenic mouse models for studying, in different organs and tissues, spontaneous or induced mutations. Such models allow study of the induction of DNA damage, repair, mutagenesis and carcinogenesis in one animal system. Accordingly, results obtained may ultimately provide greater insight into the chain of events from in vivo exposure to genotoxic agents to mutations and their ultimate physiological endpoints. In addition to their use in fundamental research, transgenic animal mutation models find a major application in the field of genetic toxicology testing, in particular with respect to organ specificity.

Nucleotide and deduced amino acid sequences of Rhizobium meliloti 102F34 lacZ gene: comparison with prokaryotic beta-galactosidases and human beta-glucuronidase

The nucleotide (nt) sequence of a 2.57-kb Sau3A fragment carrying the Rhizobium meliloti beta-galactosidase (beta Gal)-encoding gene (RmlacZ) was determined. An open reading frame (ORF) of 2.26 kb was identified which encoded a 755-amino-acid (aa) polypeptide with a calculated molecular mass of 84,141 Da, in fair agreement with the value of 88 kDa determined by SDS-PAGE. The deduced N-terminal aa sequence was confirmed by direct sequencing of electrophoretically purified R. meliloti beta Gal. The size of the native R. meliloti beta Gal was approx. 174 kDa. Similarities were found between the aa sequence of the R. meliloti beta Gal and those from Clostridium thermosulfurogenes EM1 and Agrobacterium radiobacter, as well as human beta-glucuronidase (beta Glu). Comparisons with beta Gal from Escherichia coli, Klebsiella pneumoniae, Lactobacillus bulgaricus and Kluyveromyces lactis found only weak similarities; however, the putative active site residues appear to be conserved. The RmlacZ sequence is flanked by two partially sequenced ORFs, which show aa sequence and organisational similarities to the previously reported lac operon in A. radiobacter.

Suicide-substrate inactivation of beta-galactosidase by diazomethyl beta-D-galactopyranosyl ketone

Diazomethyl beta-D-galactopyranosyl ketone (1) has been proven to be a mechanism-based, irreversible (suicide-substrate) inactivator of Aspergillus oryzae beta-D-galactosidase, but not an inactivator of E. coli lacZ beta-D-galactosidase. Compound 1 is stable in buffers of normal physiological pH. It is decomposed by H+, but not by nucleophiles. Inactivation of A. oryzae beta-D-galactopyranosyl ketone (2) nor diazomethyl alpha-D-galactopyranosyl ketone inactivated the enzyme and therefore inactivation is stereospecific, excess inhibitor could be separated from inactive enzyme without regain of activity and therefore it is bound irreversibly, and a second pulse of enzyme is inactivated at the same rate as enzyme inactivated to 95% activity by the first pulse. Diazomethyl beta-D-glucopyranosyl ketone (2) inhibited sweet almond beta-D-glucosidase.

Sequence of the Kluyveromyces lactis beta-galactosidase: comparison with prokaryotic enzymes and secondary structure analysis

The LAC4 gene encoding the beta-galactosidase (beta Gal) of the yeast, Kluyveromyces lactis, was cloned on a 7.2-kb fragment by complementation of a lacZ-deficient Escherichia coli strain. The nucleotide sequence of the structural gene, with 42 bp and 583 bp of the 5'- and 3'-flanking sequences, respectively, was determined. The deduced amino acid (aa) sequence of the K. lactis beta Gal predicts a 1025-aa polypeptide with a calculated M(r) of 117618 and reveals extended sequence homologies with all the published prokaryotic beta Gal sequences. This suggests that the eukaryotic beta Gal is closely related, evolutionarily and structurally, to the prokaryotic beta Gal's. In addition, sequence similarities were observed between the highly conserved N-terminal two-thirds of the beta Gal and the entire length of the beta-glucuronidase (beta Glu) polypeptides, which suggests that beta Glu is clearly related, structurally and evolutionarily, to the N-terminal two-thirds of the beta Gal. The structural analysis of the beta Gal alignment, performed by mean secondary structure prediction, revealed that most of the invariant residues are located in turn or loop structures. The location of the invariant residues is discussed with respect to their accessibility and their possible involvement in the catalytic process.

Cloning and analysis of the beta-galactosidase-encoding gene from Clostridium thermosulfurogenes EM1

Clostridium thermosulfurogenes EM1 produced a thermostable (up to 70 degrees C) beta-galactosidase (beta Gal) with a pH optimum of 7 during growth on lactose. The gene (lacZ) encoding this enzyme was cloned and expressed in Escherichia coli using pUC18 as a vector. The nucleotide sequence of a 2.7-kb PstI fragment carrying the lacZ gene was determined. The open reading frame for lacZ, which encoded a protein of 716 amino acids with a calculated Mr of 83,728, was confirmed by the identity of its deduced aa sequence with the chemically determined N-terminal aa sequence of the purified beta Gal of C. thermosulfurogenes EM1. The structural gene was preceded by a possible promoter sequence, 5'-TTGTAG (-35), 5'-TAATAT (-10); and a ribosome-binding site, 5'-AGGAGG. The cloned beta Gal was found to be indistinguishable from the native enzyme. The Mr of the active beta Gal was 170,000, as determined by Superose 12HR gel filtration and gradient gel electrophoresis. This indicated that this enzyme is composed of two identical subunits. Comparison of the aa sequences of different beta Gal revealed that five large regions of similarity with the enzymes from E. coli (lacZ, ebgA), Klebsiella pneumoniae (lacZ), and Lactobacillus bulgaricus are present in the beta Gal of C. thermosulfurogenes EM1 and that the putative active site residues (Glu461 and Tyr503 in the E. coli lacZ-encoded beta Gal) are conserved (Glu389 and Tyr429). Therefore, the thermostable beta Gal of C. thermosulfurogenes EM1 is more closely related to the enzyme of E. coli than to the likewise thermostable one of Bacillus stearothermophilus.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple replacements establish the importance of tyrosine-503 in beta-galactosidase (Escherichia coli)

Tyr-503 of beta-galactosidase was specifically replaced with Phe, His, Cys, and Lys using site-directed mutagenesis. The normal enzyme and the substituted enzymes were purified. The activities of each of the substituted enzymes with o-nitrophenyl-beta-D-galactopyranoside (ONPG) and p-nitrophenyl-beta-D-galactopyronoside (PNPG) were very low and Y503K-beta-galactosidase was essentially inactive, showing that Tyr-503 is important for activity. The stability (including tetrameric stability) of the enzymes at 4 and 25 degrees C was essentially the same as that of the wild-type enzyme and the cleavage patterns on sodium dodecyl sulfate gels after protease action were unchanged. These studies thus indicate that Tyr-503 has no noticeable influence on stability under normal conditions. The substitutions for Tyr-503 had some small effects on the binding of both substrate and inhibitor. However, both kappa 2 (glycosidic bond cleavage rate) and kappa 3 (hydrolysis rate constant) were dramatically reduced. Each substitution except that of Lys (which can be explained by electrostatic effects) gave decreases in kappa 2 and kappa 3 of roughly the same magnitude regardless of whether the substitutions were conservative or not. This strongly implies that the changes in rate were not due to conformational changes as it is very unlikely that there would be such similar decreases in the values of kappa 2 and kappa 3 for amino acids with such different structures and chemical properties if the changes in rate were due to conformational differences. The data suggest that one possible role of Tyr-503 is as a general acid/base catalyst. Profiles of the kinetic data of the enzymes as functions of pH supported the suggestion that Tyr-503 normally acts as a general acid and base catalyst. When Tyr-503 was substituted by His, a small amount of base catalytic activity seemed to be restored. The strongest evidence that Tyr-503 acts as an acid catalyst came from studies with isoquinolinium-beta-D-galactopyranoside as the substrate. The kappa cat(s) of Y503F-beta-galactosidase and of Y503C-beta-galactosidase decreased by about an order of magnitude while the rate decreases were about 3 orders of magnitude with ONPG and PNPG. The breakdown of isoquinolinium-beta-D-galactopyranoside cannot be catalyzed by acids.

Expression and nucleotide sequence of the Lactobacillus bulgaricus beta-galactosidase gene cloned in Escherichia coli

The Lactobacillus bulgaricus beta-galactosidase gene was cloned on a ca. 7-kilobase-pair HindIII fragment in the vector pKK223-3 and expressed in Escherichia coli by using its own promoter. The nucleotide sequence of the gene and approximately 400 bases of 3'- and 5'-flanking sequences was determined. The amino acid sequence of the beta-galactosidase, deduced from the nucleotide sequence of the gene, yielded a monomeric molecular mass of ca. 114 kilodaltons, slightly smaller than the E. coli lacZ and Klebsiella pneumoniae lacZ enzymes but larger than the E. coli evolved (ebgA) beta-galactosidase. The cloned beta-galactosidase was found to be indistinguishable from the native enzyme by several criteria. From amino acid sequence alignments, the L. bulgaricus beta-galactosidase has a 30 to 34% similarity to the E. coli lacZ, E. coli ebgA, and K. pneumoniae lacZ enzymes. There are seven regions of high similarity common to all four of these beta-galactosidases. Also, the putative active-site residues (Glu-461 and Tyr-503 in the E. coli lacZ beta-galactosidase) are conserved in the L. bulgaricus enzyme as well as in the other two beta-galactosidases mentioned above. The conservation of active-site amino acids and the large regions of similarity suggest that all four of these beta-galactosidases evolved from a common ancestral gene. However, these enzymes are quite different from the thermophilic beta-galactosidase encoded by the Bacillus stearothermophilus bgaB gene.

Identification and sequence analysis of a silent gene (ushA0) in Salmonella typhimurium

The evolution of new or improved enzyme specificities in prokaryotes has been proposed to involve gene duplication, followed by silencing of one of the duplicates at the transcriptional or translational level. Such "silent gene intermediates" are distinct from "cryptic" genes, which are proposed to have a different role in evolution. We describe the identification in Salmonella typhimurium of a silent gene (ushA0) using the active (homologous) ushA gene (encoding UDP-sugar hydrolase) from Escherichia coli as a probe. The ushA0 gene has been cloned and, in the multicopy state, very weak expression can be detected; the gene product was shown to be immunologically and functionally related to the enzyme from E. coli. The sequence of the ushA0 gene was found to be highly homologous to the previously determined sequence of the ushA gene, and the respective promoter and ribosomal-binding sites are also very similar. However, a presumed strong rho-independent terminator in the ushA gene is absent from ushA0; although a weak stem-and-loop structure is present in the 3' region of ushA0, its structure is atypical of rho-independent terminators. The sequence analysis also revealed an insertion-sequence like sequence at the 3' end of ushA0 with a convergent open reading frame terminating 116 base-pairs from the ushA0 stop codon. A deletion of the 5' region of the open reading frame results in increased expression of ushA0, indicating that convergent transcription plays some role in the silencing of ushA0. S. typhimurium contains a UDP-sugar hydrolase, biochemically and genetically distinct from that in E. coli, encoded by the ushB gene. Our results indicate that ushB is not strongly sequence-related to ushA0, and its gene product is not immunologically related to the ushA gene product. ushB is hence a functional duplicate of ushA and provides a rationale for the silencing of ushA0. This situation, and the DNA sequence comparison of ushA and ushA0, strongly suggests that rather than being a cryptic gene, ushA0 has been silenced recently during the evolution of S. typhimurium.

Structure of a beta-galactosidase gene of Bacillus stearothermophilus

The nucleotide sequence of the bgaB gene, which encodes the thermostable beta-galactosidase I of Bacillus stearothermophilus, and its flanking region was determined. A 2,016-base-pair open reading frame observed was concluded to be for beta-galactosidase I (Mr 78,051) from observations that the amino acid composition of the enzyme and the sequence of 14 amino acids from the amino-terminus of the enzyme coincided with those deduced from this open frame. A 107-base-pair HaeIII-AluI fragment just upstream of the estimated Shine-Dalgarno sequence of the bgaB gene had promoter activity toward cat-86 (chloramphenicol acetyltransferase gene) and produced the enzyme at a level equivalent to 7% of the total cellular protein of B. subtilis. From the base sequence of this DNA region and the transcriptional start site determined by S1 nuclease mapping, the -35 and -10 sequences are estimated to be TTGACA and TAATTT, respectively, which are similar to the consensus sequence of B. subtilis sigma 43 RNA polymerase.