Bovine fetal microchimerism in normal and embryo transfer pregnancies and its implications for biotechnology applications in cattle

Fetal cells and DNA have been detected in the maternal circulation during and after pregnancy in a few mammalian species. The incidence of similar microchimerism in cattle could have repercussion for the application of modern biotechnologies such as the transfer of transgenic embryos. To determine if feto-maternal leakage can occur in pregnant cows, we have analyzed maternal blood samples for the presence of fetal DNA during gestation and post-partum periods. Y chromosome-specific DNA was detected in up to 73% of blood samples from naturally mated heifers carrying conventional bull calves and a transgene-specific sequence in up to 50% of recipient cows carrying transgenic fetuses. These findings document for the first time that transplacental leakage of fetal DNA into the maternal circulation can occur in cattle despite the epitheliochorial placenta of ruminants, with potential implications for the utilization of recipient cows in the food chain.

Compositional analysis of dairy products derived from clones and cloned transgenic cattle

Cloning technology is an emerging biotechnological tool that could provide commercial opportunities for livestock agriculture. However, the process is very inefficient and the molecular events underlying the technology are poorly understood. The resulting uncertainties are causing concerns regarding the safety of food products derived from cloned livestock. There are similar concerns for livestock produced by biotechnologies which enable the purposeful introduction of genetic modifications. To increase the knowledge about food products from animals generated by these modern biotechnologies, we assessed compositional differences associated with milk and cheese derived from cloned and transgenic cows. Based on gross composition, fatty acid and amino acid profiles and mineral and vitamin contents, milk produced by clones and conventional cattle were essentially similar and consistent with reference values from dairy cows farmed in the same region under similar conditions. Whereas colostrum produced by transgenic cows with additional casein genes had similar IgG secretion levels and kinetics to control cows, milk from the transgenic cows had a distinct yellow appearance, in contrast to the white color of milk from control cows. Processing of milk into cheese resulted in differences in the gross composition and amino acid profiles; 'transgenic' cheese had lower fat and higher salt contents and small but characteristic differences in the amino acid profile compared to control cheese.

Oligonucleotide-mediated gene modification and its promise for animal agriculture

One of the great aspirations in modern biology is the ability to utilise the expanding knowledge of the genetic basis of phenotypic diversity through the purposeful tailoring of the mammalian genome. A number of technologies are emerging which have the capacity to modify genes in their chromosomal context. Not surprisingly, the major thrust in this area has come from the evaluation of gene therapy applications to correct mutations implicated in human genetic diseases. The recent development of somatic cell nuclear transfer (SCNT) provides access to these technologies for the purposeful modification of livestock animals. The enormous phenotypic variety existent in contemporary livestock animals has in many cases been linked to quantitative trait loci (QTL) and their underlying point mutations, often referred to as single-nucleotide polymorphisms (SNPs). Thus, the ability for the targeted genetic modification of livestock animals constitutes an attractive opportunity for future agricultural applications. In this review, we will summarize attempts and approaches for oligonucleotide-mediated gene modification (OGM) strategies for the site-specific modification of the genome, with an emphasis on chimeric RNA-DNA oligonucleotides (RDOs) and single-stranded oligonucletides (ssODNs). The potential of this approach for the directed genetic improvement of livestock animals is illustrated through examples, outlining the effects of point mutations on important traits, including meat and milk production, reproductive performance, disease resistance and superior models of human diseases. Current technological hurdles and potential strategies that might remove these barriers in the future are discussed.

330 UTILIZING PRE-IMPLANTATION GENETIC DIAGNOSIS AND OPU-IVP-ET TO GENERATE MULTIPLE PROGENY OF PREDETERMINED GENOTYPE FROM CLONED TRANSGENIC HEIFERS

Advanced reproductive technologies, comprising ovum pickup (OPU), in vitro embryo production (IVP), and embryo transfer (ET), were used to rapidly generate multiple F1 offspring from nine hemizygous founder-cloned transgenic (TG) heifers that highly express casein protein in their milk (Brophy B et al. 2003 Nat. Biotech. 21, 157–62). Moreover, embryo biopsy and genotyping were used to determine gender and presence or absence of the additional casein genes prior to ET. This minimized the number of surplus animals that would otherwise be generated from the breeding program. The aim was to generate 10 TG female (TG-F), 10 non-TG female (non-TG-F), two TG male (TG-M), and two non-TG male (non-TG-M) offspring. Some hemizygous TG bulls were required to enable the generation of homozygous transgenic animals in the next generation, while non-TG cattle would serve as controls in future studies. Transvaginal OPU was performed on non-stimulated TG heifers twice weekly for 6 weeks. Selected oocytes were matured, then fertilized in vitro (Day 0) with sperm from a conventional sire, and cultured to Day 6 (Thompson JG et al. 2000 J. Reprod. Fert. 118, 47–55). Transferable quality embryos were biopsied and cultured singularly for another 20 h, by which time genotyping results were available. On Day 7, individual embryos of the desired genotype were identified and transferred nonsurgically to synchronized recipients. Two biopsy methods were used, dependent on the stage of the embryo. For blastocysts, 10–12 trophectoderm cells were biopsied using an ultra-sharp splitting blade mounted on a micromanipulator (Herr CM et al. 1991 Theriogenology 35, 45–54). For morulae, the zonae pellucidae were enzymatically removed and two blastomeres per embryo aspirated with a blunt 30-μm micropipette (Chrenek P et al. 2001 Theriogenology 55, 1071–81). Aliquots either from the lysed blastocyst biopsy samples or following primer extension pre-amplification of DNA (Zhang L et al. 1992 Proc. Natl. Acad. Sci. USA 89, 5847–51) from blastomere samples were used to determine sex and casein transgene status using multiplex PCR. Following the OPU program, the TG heifers were artificially inseminated with semen from the same sire as used for IVF; eight calved (89%). Data were compared using Fisher's exact test. The mean number of transferable Day 6 embryos produced per donor per OPU session was 1.3. Pregnancy rates following OPU-IVP-ET on Day 35 were 27/62 (44%) with a calving rate of 29%. Nine TG-F, five non-TG-F, three TG-M, and one non-TG-M were delivered vaginally at term. All calves born were of the predicted genotype. There was no significant difference in embryonic survival to term either between TG and non-TG embryos transferred, or between biopsy methods. One OPU calf died shortly after birth; the other 17 survived to weaning, as did all the AI calves. The results show that pre-implantation genetic diagnosis combined with OPU-IVP-ET can be successfully used to produce small scale-up herds from founder TG females.

Cloned cattle derived from a novel zona-free embryo reconstruction system

As the demand for cloned embryos and offspring increases, the need arises for the development of nuclear transfer procedures that are improved in both efficiency and ease of operation. Here, we describe a novel zona-free cloning method that doubles the throughput in cloned bovine embryo production over current procedures and generates viable offspring with the same efficiency. Elements of the procedure include zona-free enucleation without a holding pipette, automated fusion of 5-10 oocyte-donor cell pairs and microdrop in vitro culture. Using this system, zona-free embryos were reconstructed from five independent primary cell lines and cultured either singularly (single-IVC) or as aggregates of three (triple-IVC). Blastocysts of transferable quality were obtained at similar rates from zona-free single-IVC, triple-IVC, and control zona-intact embryos (33%, 25%, and 29%, respectively). In a direct comparison, there was no significant difference in development to live calves at term between single-IVC, triple-IVC, and zona-intact embryos derived from the same adult fibroblast line (10%, 13%, and 15%, respectively). This zona-free cloning method could be straightforward for users of conventional cloning procedures to adopt and may prove a simple, fast, and efficient alternative for nuclear cloning of other species as well.

Cloned transgenic cattle produce milk with higher levels of beta-casein and kappa-casein

To enhance milk composition and milk processing efficiency by increasing the casein concentration in milk, we have introduced additional copies of the genes encoding bovine beta- and kappa-casein (CSN2 and CSN3, respectively) into female bovine fibroblasts. Nuclear transfer with four independent donor cell lines resulted in the production of 11 transgenic calves. The analysis of hormonally induced milk showed substantial expression and secretion of the transgene-derived caseins into milk. Nine cows, representing two high-expressing lines, produced milk with an 8-20% increase in beta-casein, a twofold increase in kappa-casein levels, and a markedly altered kappa-casein to total casein ratio. These results show that it is feasible to substantially alter a major component of milk in high producing dairy cows by a transgenic approach and thus to improve the functional properties of dairy milk.

Coordination between donor cell type and cell cycle stage improves nuclear cloning efficiency in cattle

Several studies have shown that both quiescent and proliferating somatic donor cells can be fully reprogrammed after nuclear transfer (NT) and result in viable offspring. So far, however, no comparative study has conclusively demonstrated the relative importance of donor cell cycle stage on nuclear cloning efficiency. Here, we compare two different types of bovine fetal fibroblasts (BFFs) that were synchronized in G(0), G(1), and different phases within G(1). We show that for non-transgenic (non-TG) fibroblasts, serum starvation into G(0) results in a significantly higher percentage of viable calves at term than synchronization in early G(1) or late G(1). For transgenic fibroblasts, however, cells selected in G(1) show significantly higher development to calves at term and higher post-natal survival to weaning than cells in G(0). This suggests that it may be necessary to coordinate donor cell type and cell cycle stage to maximize overall cloning efficiency.

Over-expression of the SUV39H1 histone methyltransferase induces altered proliferation and differentiation in transgenic mice

The development of multi-cellular organisms is regulated by the ordered definition of gene expression programmes that govern cell proliferation and differentiation. Although differential gene activity is mainly controlled by transcription factors, it is also dependent upon the underlying chromatin structure, which can stabilize transcriptional "on" or "off" states. We have recently isolated human (SUV39H1) and mouse (Suv39h1) histone methyltransferases (HMTases) and shown that they are important regulators for the organization of repressive chromatin domains. To investigate whether a SUV39H1-induced modulation of heterochromatin would affect mammalian development, we generated transgenic mice that over-express the SUV39H1 HMTase early during embryogenesis. SUV39H1 transgenic mice are growth retarded, display a weak penetrance of skeletal transformations and are largely characterized by impaired erythroid differentiation, consistent with highest transgene expression in foetal liver. Ex vivo transgenic foetal liver cultures initially contain reduced numbers of cells in G1 but progress to immortalized erythroblasts that are compromised in executing an erythroid differentiation programme. The outgrowing SUV39H1-immortalized erythroblasts can maintain a diploid karyotype despite deregulation of several tumour suppressor proteins and dispersed distribution of the heterochromatin component HP1. Together, these data provide evidence for a role of the SUV39H1 HMTase during the mammalian development and indicate a possible function for higher-order chromatin in contributing to the balance between proliferation and differentiation potentials of progenitor cells.

Isolation and characterization of Suv39h2, a second histone H3 methyltransferase gene that displays testis-specific expression

Higher-order chromatin has been implicated in epigenetic gene control and in the functional organization of chromosomes. We have recently discovered mouse (Suv39h1) and human (SUV39H1) histone H3 lysine 9-selective methyltransferases (Suv39h HMTases) and shown that they modulate chromatin dynamics in somatic cells. We describe here the isolation, chromosomal assignment, and characterization of a second murine gene, Suv39h2. Like Suv39h1, Suv39h2 encodes an H3 HMTase that shares 59% identity with Suv39h1 but which differs by the presence of a highly basic N terminus. Using fluorescent in situ hybridization and haplotype analysis, the Suv39h2 locus was mapped to the subcentromeric region of mouse chromosome 2, whereas the Suv39h1 locus resides at the tip of the mouse X chromosome. Notably, although both Suv39h loci display overlapping expression profiles during mouse embryogenesis, Suv39h2 transcripts remain specifically expressed in adult testes. Immunolocalization of Suv39h2 protein during spermatogenesis indicates enriched distribution at the heterochromatin from the leptotene to the round spermatid stage. Moreover, Suv39h2 specifically accumulates with chromatin of the sex chromosomes (XY body) which undergo transcriptional silencing during the first meiotic prophase. These data are consistent with redundant enzymatic roles for Suv39h1 and Suv39h2 during mouse development and suggest an additional function of the Suv39h2 HMTase in organizing meiotic heterochromatin that may even impart an epigenetic imprint to the male germ line.

Structure-function analysis of SUV39H1 reveals a dominant role in heterochromatin organization, chromosome segregation, and mitotic progression

SUV39H1, a human homologue of the Drosophila position effect variegation modifier Su(var)3-9 and of the Schizosaccharomyces pombe silencing factor clr4, encodes a novel heterochromatic protein that transiently accumulates at centromeric positions during mitosis. Using a detailed structure-function analysis of SUV39H1 mutant proteins in transfected cells, we now show that deregulated SUV39H1 interferes at multiple levels with mammalian higher-order chromatin organization. First, forced expression of full-length SUV39H1 (412 amino acids) redistributes endogenous M31 (HP1beta) and induces abundant associations with inter- and metaphase chromatin. These properties depend on the C-terminal SET domain, although the major portion of the SUV39H1 protein (amino acids 89 to 412) does not display affinity for nuclear chromatin. By contrast, the M31 interaction surface, which was mapped to the first 44 N-terminal amino acids, together with the immediately adjacent chromo domain, directs specific accumulation at heterochromatin. Second, cells overexpressing full-length SUV39H1 display severe defects in mitotic progression and chromosome segregation. Surprisingly, whereas localization of centromere proteins is unaltered, the focal, G(2)-specific distribution of phosphorylated histone H3 at serine 10 (phosH3) is dispersed in these cells. This phosH3 shift is not observed with C-terminally truncated mutant SUV39H1 proteins or with deregulated M31. Together, our data reveal a dominant role(s) for the SET domain of SUV39H1 in the distribution of prominent heterochromatic proteins and suggest a possible link between a chromosomal SU(VAR) protein and histone H3.

Functional mammalian homologues of the Drosophila PEV-modifier Su(var)3-9 encode centromere-associated proteins which complex with the heterochromatin component M31

The chromo and SET domains are conserved sequence motifs present in chromosomal proteins that function in epigenetic control of gene expression, presumably by modulating higher order chromatin. Based on sequence information from the SET domain, we have isolated human (SUV39H1) and mouse (Suv39h1) homologues of the dominant Drosophila modifier of position-effect-variegation (PEV) Su(var)3-9. Mammalian homologues contain, in addition to the SET domain, the characteristic chromo domain, a combination that is also preserved in the Schizosaccharyomyces pombe silencing factor clr4. Chromatin-dependent gene regulation is demonstrated by the potential of human SUV39H1 to increase repression of the pericentromeric white marker gene in transgenic flies. Immunodetection of endogenous Suv39h1/SUV39H1 proteins in a variety of mammalian cell lines reveals enriched distribution at heterochromatic foci during interphase and centromere-specific localization during metaphase. In addition, Suv39h1/SUV39H1 proteins associate with M31, currently the only other characterized mammalian SU(VAR) homologue. These data indicate the existence of a mammalian SU(VAR) complex and define Suv39h1/SUV39H1 as novel components of mammalian higher order chromatin.

SET domain proteins modulate chromatin domains in eu- and heterochromatin

The SET domain is a 130-amino acid, evolutionarily conserved sequence motif present in chromosomal proteins that function in modulating gene activities from yeast to mammals. Initially identified as members of the Polycomb- and trithorax-group (Pc-G and trx-G) gene families, which are required to maintain expression boundaries of homeotic selector (HOM-C) genes, SET domain proteins are also involved in position-effect-variegation (PEV), telomeric and centromeric gene silencing, and possibly in determining chromosome architecture. These observations implicate SET domain proteins as multifunctional chromatin regulators with activities in both eu- and heterochromatin--a role consistent with their modular structure, which combines the SET domain with additional sequence motifs of either a cysteine-rich region/zinc-finger type or the chromo domain. Multiple functions for chromatin regulators are not restricted to the SET protein family, since many trx-G (but only very few Pc-G) genes are also modifiers of PEV. Together, these data establish a model in which the modulation of chromatin domains is mechanistically linked with the regulation of key developmental loci (e.g. HOM-C).

Characterization of a gene encoding a DNA-binding protein that interacts in vitro with vascular specific cis elements of the phenylalanine ammonia-lyase promoter

A study of the expression of a bean phenylalanine ammonia-lyase (PAL) promoter/beta-glucuronidase gene fusion in transgenic tobacco has shown that the PAL2 promoter has a modular organization. Expression of the PAL2 promoter in the vascular system involves positive and negative regulatory cis elements. Among these elements is an AC-rich motif implicated in xylem expression and a suppressing cis element for phloem expression. Using radiolabelled complementary oligonucleotides bearing the AC-rich motif, a cDNA clone encoding a DNA-binding protein has been isolated from a tobacco lambda gt11 expression library. This factor, named AC-rich binding factor (ACBF), showed binding specificity to the AC-rich region. The specificity of ACBF for the AC-rich region was also shown using a gel retardation assay with an ACBF recombinant protein extract. The deduced amino acid sequence from ACBF contains a long repeat of glutamine residues as found in well characterized transcription factors. Interestingly, ACBF shared sequence similarity to conserved amino acid motifs found in RNA-binding proteins. Genomic gel blot analysis indicated the presence of a small gene family of sequences related to ACBF within the tobacco nuclear genome. Analysis of tobacco mRNA using the ACBF cDNA as probe showed that while ACBF mRNA was present in all tissues examined, the highest transcript accumulation occurred in stem tissues. The functional characteristics of the AC-rich sequence were examined in transgenic tobacco. A heptamer of the AC-rich sequence, in front of a minimal 35S promoter from cauliflower mosaic virus (-46 to +4), conferred specific expression in xylem.

Mammalian homologues of the Polycomb-group gene Enhancer of zeste mediate gene silencing in Drosophila heterochromatin and at S. cerevisiae telomeres

Gene silencing is required to stably maintain distinct patterns of gene expression during eukaryotic development and has been correlated with the induction of chromatin domains that restrict gene activity. We describe the isolation of human (EZH2) and mouse (Ezh1) homologues of the Drosophila Polycomb-group (Pc-G) gene Enhancer of zeste [E(z)], a crucial regulator of homeotic gene expression implicated in the assembly of repressive protein complexes in chromatin. Mammalian homologues of E(z) are encoded by two distinct loci in mouse and man, and the two murine Ezh genes display complementary expression profiles during mouse development. The E(z) gene family reveals a striking functional conservation in mediating gene repression in eukaryotic chromatin: extra gene copies of human EZH2 or Drosophila E(z) in transgenic flies enhance position effect variegation of the heterochromatin-associated white gene, and expression of either human EZH2 or murine Ezh1 restores gene repression in Saccharomyces cerevisiae mutants that are impaired in telomeric silencing. Together, these data provide a functional link between Pc-G-dependent gene repression and inactive chromatin domains, and indicate that silencing mechanism(s) may be broadly conserved in eukaryotes.

Extension of chromatin accessibility by nuclear matrix attachment regions

Transcription of the variable region of the rearranged immunoglobulin mu gene is dependent on an enhancer sequence situated within one of the introns of the gene. Experiments with transgenic mice have shown that activation of the promoter controlling this transcription also requires the matrix-attachment regions (MARs) that flank the intronic enhancer. As this mu gene enhancer can establish local areas of accessible chromatin, we investigated whether the MARs can extend accessibility to more distal positions. We eliminated interactions between enhancer- and promoter-bound factors by linking mu enhancer/MAR fragments to the binding sites for bacteriophage RNA polymerases that were either close to or one kilobase distal to the enhancer. The mu enhancer alone mediated chromatin accessibility at the proximal site but required a flanking MAR to confer accessibility upon the distal promoter. This long-range accessibility correlates with extended demethylation of the gene construct but not with whether it is being actively transcribed. MARs thus collaborate with the mu enhancer to generate an extended domain of accessible chromatin.

Crystallization of a genetically engineered water-soluble primary penicillin target enzyme. The high molecular mass PBP2x of Streptococcus pneumoniae

A genetically engineered water-soluble derivative of PBP2x of Streptococcus pneumoniae has been produced, purified and crystallized in a form suitable for X-ray diffraction analysis. The best crystals have been grown at 15 degrees C, from solutions containing 8% polyethylene glycol 10,000 at pH values ranging from 3.9 to 6.0. These crystals diffract to a resolution of 3.5 A and have a space group P6(1)22 (or enantiomorph) with unit cell dimensions of a = b = 162.2 A, c = 171.8 A, alpha = beta = 90 degrees, gamma = 120 degrees. The molecular mass and cell dimensions suggest that there is one molecule of enzyme per asymmetric unit. The breakdown of a chromogenic cephalosporin derivative diffused into a crystal reveals clearly that the enzyme is active in the crystalline state.

Penicillin-binding protein 2x of Streptococcus pneumoniae. Expression in Escherichia coli and purification of a soluble enzymatically active derivative

A 2.5-kb DNA fragment including the structural gene coding for the penicillin-binding protein 2x (PBP 2x) of Streptococcus pneumoniae has been cloned into the vector pJDC9 and expressed in Escherichia coli. Mapping of RNA polymerase binding sites by electron microscopy indicated that the pbpX promoter is well recognized by the E. coli enzyme. However, high-level expression occurred mainly under the control of the lac promoter upstream of the pJDC9 multiple cloning site. After induction with isopropyl beta-d-thiogalactopyranoside, PBP 2x was expressed as one of the major cellular proteins. PBP 2x produced in E. coli corresponded to the pneumococcal PBP 2x in terms of electrophoretic mobility, fractionation with the cytoplasmic membrane, and penicillin-binding capacity. Deletion of 30 hydrophobic N-terminal amino acid residues at positions 19-48 resulted in high-level expression of a cytoplasmic, soluble PBP 2x derivative (PBP 2x*) which still retained full beta-lactam-binding activity. A two-step procedure involving dye affinity chromatography was established for obtaining large amounts of highly purified enzymatically active PBP 2x*.

Five independent combinations of mutations can result in low-affinity penicillin-binding protein 2x of Streptococcus pneumoniae

Penicillin-binding protein 2x (PBP 2x) of Streptococcus pneumoniae is one of the high-molecular-weight PBPs involved in the development of intrinsic beta-lactam resistance. Point mutations in the PBP 2x genes (pbpX) have now been characterized in five independent spontaneous laboratory mutants in order to identify protein regions which are important for interaction with beta-lactam antibiotics. All mutant genes contained two to four mutations resulting in amino acid substitutions within the penicillin-binding domain of PBP 2x, and none of the mutants carried an identical set of mutations. For one particular mutant, C606, carrying four mutations in pbpX, the mutations at positions 601 and 597 conferred first- and second-level resistance when introduced into the susceptible parent strain S. pneumoniae R6. However, the other two mutations, at amino acid positions 289 and 422, which were originally selected at the fifth and sixth isolation steps, did not contribute at all to resistance in similar experiments. This suggests that they are phenotypically expressed only in combination with mutations in other genes. Three PBP 2x regions were mutated in from two to all four mutants carrying a low-affinity PBP 2x. However, in a fifth mutant containing a PBP 2x with apparent zero affinity for beta-lactams, the three mutations in pbpX mapped at entirely different positions. This demonstrates that different mutational pathways exist for remodeling this PBP during resistance development.

Interspecies recombinational events during the evolution of altered PBP 2x genes in penicillin-resistant clinical isolates of Streptococcus pneumoniae

Penicillin resistance in pneumococci is due to the appearance of high molecular-weight penicillin-binding proteins (PBPs) that have reduced affinity for the antibiotic. We have compared the PBX 2x genes (pbpX) of one penicillin-susceptible and five penicillin-resistant clinical isolates of Streptococcus pneumoniae isolated from various parts of the world. All of the resistant isolates contained a low-affinity form of PBP 2x. The 2 kb region of the two penicillin-susceptible isolates differed at only eight nucleotide sites (0.4%) and resulted in one single amino acid difference in PBP 2x. In contrast, the sequences of the PBP 2x genes from the resistant isolates differed overall from those of the susceptible isolates at between 7 and 18% of nucleotide sites and resulted in between 27 and 86 amino acid substitutions in PBP 2x. The altered PBP 2x genes consisted of regions that were similar to those of susceptible strains (less than 3% diverged), alternating with regions that were very different (18-23% diverged). The presence of highly diverged regions within the PBP 2x genes of the resistant isolates contrasts with the uniformity of the sequences of the amylomaltase genes from the same isolates, and with the uniformity of the PBP 2x genes in the two susceptible isolates. It suggests that the altered PBP 2x genes have arisen by localized interspecies recombinational events involving the PBP 2x genes of closely related streptococci, as has been suggested to occur for altered PBP 2b genes (Dowson et al., 1989b). The PBP 2x genes from the resistant isolates could transform the susceptible strain R6 to increased levels of resistance to beta-lactam antibiotics, indicating that the altered forms of PBP 2x in the resistant isolates contribute to their resistance to penicillin.

Intercontinental spread of a multiresistant clone of serotype 23F Streptococcus pneumoniae

Isolates of serotype 23F Streptococcus pneumoniae with high levels of resistance of penicillin have been commonly recovered in Spain for more than a decade. Recently penicillin-resistant serotype 23F S. pneumoniae strains were also isolated from children attending a day-care center in Cleveland. A number of Spanish and Cleveland isolates were compared by electrophoretic analysis of penicillin-binding protein (PBP) profiles and DNA restriction endonuclease cleavage profiles of the PBP 2X and 2B genes amplified with the polymerase chain reaction and by multilocus enzyme electrophoresis. All strains were identical by these criteria. The findings demonstrate that the Spanish and Cleveland isolates are clonally related and suggest that this antibiotic resistant clone of serotype 23F S. pneumoniae has spread intercontinentally from Spain to the United States.

Penicillin-binding proteins in Streptococcus pneumoniae: alterations during development of intrinsic penicillin resistance

Four out of the five high molecular weight penicillin-binding proteins (PBPs) of Streptococcus pneumoniae are involved in the development of intrinsic penicillin resistance. In beta-lactam resistant laboratory mutants, point mutations in the PBP 2x-genes were identified that result in low penicillin-affinity mutant proteins. In contrast, PBPs 1a, 2x, and 2b of resistant clinical isolates are highly altered as can be recognized biochemically and immunologically; DNA sequence analysis of the PBP 2x gene from resistant strains confirmed these results. The variability of the three PBPs analyzed implies a very heterogeneous gene pool accessible to the pneumococcus that is used for recruitment of resistant PBP genes in wild type strains.