Promotion and limitation of genetic exchange

The Dawn of Plant Molecular Biology: How Three Key Methodologies Paved the Way

The adoption of Arabidopsis thaliana in the 1980s as a universal plant model finally enabled researchers to adopt and take full advantage of the molecular biology tools and methods developed in the bacterial and animal fields since the early 1970s. It further brought the plant sciences up to speed with other research fields, which had been employing widely accepted model organisms for decades. In parallel with this major development, the concurrent establishment of the plant transformation methodology and the description of the cauliflower mosaic virus (CaMV) 35S promoter enabled scientists to create robust transgenic plant lines for the first time, thereby providing a valuable tool for studying gene function. The ability to create transgenic plants launched the plant biotechnology sector, with Monsanto and Plant Genetic Systems developing the first herbicide- and pest-tolerant plants, initiating a revolution in the agricultural industry. Here I review the major developments over a less than 10-year span and demonstrate how they complemented each other to trigger a revolution in plant molecular biology and launch an era of unprecedented progress for the whole plant field. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

The rotation-coupled sliding of EcoRV

It has been proposed that certain type II restriction enzymes (REs), such as EcoRV, track the helical pitch of DNA as they diffuse along DNA, a so-called rotation-coupled sliding. As of yet, there is no direct experimental observation of this phenomenon, but mounting indirect evidence gained from single-molecule imaging of RE-DNA complexes support the hypothesis. We address this issue by conjugating fluorescent labels of varying size (organic dyes, proteins and quantum dots) to EcoRV, and by fusing it to the engineered Rop protein scRM6. Single-molecule imaging of these modified EcoRVs sliding along DNA provides us with their linear diffusion constant (D(1)), revealing a significant size dependency. To account for the dependence of D(1) on the size of the EcoRV label, we have developed four theoretical models describing different types of motion along DNA and find that our experimental results are best described by rotation-coupled sliding of the protein. The similarity of EcoRV to other type II REs and DNA binding proteins suggests that this type of motion could be widely preserved in other biological contexts.

Evolutionary dynamics of clustered irregularly interspaced short palindromic repeat systems in the ocean metagenome

Clustered regularly interspaced short palindromic repeats (CRISPRs) form a recently characterized type of prokaryotic antiphage defense system. The phage-host interactions involving CRISPRs have been studied in experiments with selected bacterial or archaeal species and, computationally, in completely sequenced genomes. However, these studies do not allow one to take prokaryotic population diversity and phage-host interaction dynamics into account. This gap can be filled by using metagenomic data: in particular, the largest existing data set, generated from the Sorcerer II Global Ocean Sampling expedition. The application of three publicly available CRISPR recognition programs to the Global Ocean metagenome produced a large proportion of false-positive results. To address this problem, a filtering procedure was designed. It resulted in about 200 reliable CRISPR cassettes, which were then studied in detail. The repeat consensuses were clustered into several stable classes that differed from the existing classification. Short fragments of DNA similar to the cassette spacers were more frequently present in the same geographical location than in other locations (P, <0.0001). We developed a catalogue of elementary CRISPR-forming events and reconstructed the likely evolutionary history of cassettes that had common spacers. Metagenomic collections allow for relatively unbiased analysis of phage-host interactions and CRISPR evolution. The results of this study demonstrate that CRISPR cassettes retain the memory of the local virus population at a particular ocean location. CRISPR evolution may be described using a limited vocabulary of elementary events that have a natural biological interpretation.

The role of interactions between phage and bacterial proteins within the infected cell: a diverse and puzzling interactome

Interactions between bacteriophage proteins and bacterial proteins are important for efficient infection of the host cell. The phage proteins involved in these bacteriophage-host interactions are often produced immediately after infection. A survey of the available set of published bacteriophage-host interactions reveals the targeted host proteins are inhibited, activated or functionally redirected by the phage protein. These interactions protect the bacteriophage from bacterial defence mechanisms or adapt the host-cell metabolism to establish an efficient infection cycle. Regrettably, a large majority of bacteriophage early proteins lack any identified function. Recent research into the antibacterial potential of bacteriophage-host interactions indicates that phage early proteins seem to target a wide variety of processes in the host cell - many of them non-essential. Since a clear understanding of such interactions may become important for regulations involving phage therapy and in biotechnological applications, increased scientific emphasis on the biological elucidation of such proteins is warranted.

Creation of a type IIS restriction endonuclease with a long recognition sequence

Type IIS restriction endonucleases cleave DNA outside their recognition sequences, and are therefore particularly useful in the assembly of DNA from smaller fragments. A limitation of type IIS restriction endonucleases in assembly of long DNA sequences is the relative abundance of their target sites. To facilitate ligation-based assembly of extremely long pieces of DNA, we have engineered a new type IIS restriction endonuclease that combines the specificity of the homing endonuclease I-SceI with the type IIS cleavage pattern of FokI. We linked a non-cleaving mutant of I-SceI, which conveys to the chimeric enzyme its specificity for an 18-bp DNA sequence, to the catalytic domain of FokI, which cuts DNA at a defined site outside the target site. Whereas previously described chimeric endonucleases do not produce type IIS-like precise DNA overhangs suitable for ligation, our chimeric endonuclease cleaves double-stranded DNA exactly 2 and 6 nt from the target site to generate homogeneous, 5', four-base overhangs, which can be ligated with 90% fidelity. We anticipate that these enzymes will be particularly useful in manipulation of DNA fragments larger than a thousand bases, which are very likely to contain target sites for all natural type IIS restriction endonucleases.

Molecular analysis of restriction endonuclease EcoRII from Escherichia coli reveals precise regulation of its enzymatic activity by autoinhibition

Bacterial restriction endonuclease EcoRII requires two recognition sites to cleave DNA. Proteolysis of EcoRII revealed the existence of two stable domains, EcoRII-N and EcoRII-C. Reduction of the enzyme to its C-terminal domain, EcoRII-C, unleashed the enzyme activity; this truncated form no longer needed two recognition sites and cleaved DNA much more efficiently than EcoRII wild-type. The crystal structure of EcoRII showed that probably the N-terminal domain sterically occludes the catalytic site, thus apparently controlling the cleavage activity. Based on these data, EcoRII was the first restriction endonuclease for which an autoinhibition mechanism as regulatory strategy was proposed. In this study, we probed this assumption and searched for the inhibitory element that mediates autoinhibition. Here we show that repression of EcoRII-C is achieved by addition of the inhibitory domain EcoRII-N or by single soluble peptides thereof in trans. Moreover, we perturbed contacts between the N- and the C-terminal domain of EcoRII by site-directed mutagenesis and proved that beta-strand B1 and alpha-helix H2 are essential for autoinhibition; deletion of either secondary structural element completely relieved EcoRII autoinhibition. This potent regulation principle that keeps EcoRII enzyme activity controlled might protect bacteria against suicidal restriction of rare unmodified recognition sites in the cellular genome.

Phylogeny in aid of the present and novel microbial lineages: diversity in Bacillus

Bacillus represents microbes of high economic, medical and biodefense importance. Bacillus strain identification based on 16S rRNA sequence analyses is invariably limited to species level. Secondly, certain discrepancies exist in the segregation of Bacillus subtilis strains. In the RDP/NCBI databases, out of a total of 2611 individual 16S rDNA sequences belonging to the 175 different species of the genus Bacillus, only 1586 have been identified up to species level. 16S rRNA sequences of Bacillus anthracis (153 strains), B. cereus (211 strains), B. thuringiensis (108 strains), B. subtilis (271 strains), B. licheniformis (131 strains), B. pumilus (83 strains), B. megaterium (47 strains), B. sphaericus (42 strains), B. clausii (39 strains) and B. halodurans (36 strains) were considered for generating species-specific framework and probes as tools for their rapid identification. Phylogenetic segregation of 1121, 16S rDNA sequences of 10 different Bacillus species in to 89 clusters enabled us to develop a phylogenetic frame work of 34 representative sequences. Using this phylogenetic framework, 305 out of 1025, 16S rDNA sequences presently classified as Bacillus sp. could be identified up to species level. This identification was supported by 20 to 30 nucleotides long signature sequences and in silico restriction enzyme analysis specific to the 10 Bacillus species. This integrated approach resulted in identifying around 30% of Bacillus sp. up to species level and revealed that B. subtilis strains can be segregated into two phylogenetically distinct groups, such that one of them may be renamed.

The Fidelity Index provides a systematic quantitation of star activity of DNA restriction endonucleases

Restriction endonucleases are the basic tools of molecular biology. Many restriction endonucleases show relaxed sequence recognition, called star activity, as an inherent property under various digestion conditions including the optimal ones. To quantify this property we propose the concept of the Fidelity Index (FI), which is defined as the ratio of the maximum enzyme amount showing no star activity to the minimum amount needed for complete digestion at the cognate recognition site for any particular restriction endonuclease. Fidelity indices for a large number of restriction endonucleases are reported here. The effects of reaction vessel, reaction volume, incubation mode, substrate differences, reaction time, reaction temperature and additional glycerol, DMSO, ethanol and Mn(2+) on the FI are also investigated. The FI provides a practical guideline for the use of restriction endonucleases and defines a fundamental property by which restriction endonucleases can be characterized.

Common patterns in type II restriction enzyme binding sites

Restriction enzymes are among the best studied examples of DNA binding proteins. In order to find general patterns in DNA recognition sites, which may reflect important properties of protein–DNA interaction, we analyse the binding sites of all known type II restriction endonucleases. We find a significantly enhanced GC content and discuss three explanations for this phenomenon. Moreover, we study patterns of nucleotide order in recognition sites. Our analysis reveals a striking accumulation of adjacent purines (R) or pyrimidines (Y). We discuss three possible reasons: RR/YY dinucleotides are characterized by (i) stronger H-bond donor and acceptor clusters, (ii) specific geometrical properties and (iii) a low stacking energy. These features make RR/YY steps particularly accessible for specific protein–DNA interactions. Finally, we show that the recognition sites of type II restriction enzymes are underrepresented in host genomes and in phage genomes.

BanAI a new isoschizomer of the type II restriction endonuclease HaeIII discovered in a Bacillus anthracis isolate from Amazon Basin

Bacillus anthracis was isolated and identified from a bacterial collection of samples from the Amazon river bank. Type II restriction endonuclease activity was detected in this prokaryote, the enzyme was purified, the molecular mass of the native protein estimated by gel filtration, and optima pH, temperature and salt requirements were determined. Quality control assays showed complete absence of 'non-specific nucleases'. Restriction cleavage analysis and DNA sequencing of restriction fragments allowed unequivocal demonstration of 5'-GG downward arrow CC-3' as the recognition sequence. This enzyme was named BanAI and is therefore an isoschizomer of the prototype restriction endonuclease HaeIII. This is the first report of a type II restriction endonuclease identified, purified from a natural isolate of B. anthracis.

Structure and function of type II restriction endonucleases

More than 3000 type II restriction endonucleases have been discovered. They recognize short, usually palindromic, sequences of 4-8 bp and, in the presence of Mg(2+), cleave the DNA within or in close proximity to the recognition sequence. The orthodox type II enzymes are homodimers which recognize palindromic sites. Depending on particular features subtypes are classified. All structures of restriction enzymes show a common structural core comprising four beta-strands and one alpha-helix. Furthermore, two families of enzymes can be distinguished which are structurally very similar (EcoRI-like enzymes and EcoRV-like enzymes). Like other DNA binding proteins, restriction enzymes are capable of non-specific DNA binding, which is the prerequisite for efficient target site location by facilitated diffusion. Non-specific binding usually does not involve interactions with the bases but only with the DNA backbone. In contrast, specific binding is characterized by an intimate interplay between direct (interaction with the bases) and indirect (interaction with the backbone) readout. Typically approximately 15-20 hydrogen bonds are formed between a dimeric restriction enzyme and the bases of the recognition sequence, in addition to numerous van der Waals contacts to the bases and hydrogen bonds to the backbone, which may also be water mediated. The recognition process triggers large conformational changes of the enzyme and the DNA, which lead to the activation of the catalytic centers. In many restriction enzymes the catalytic centers, one in each subunit, are represented by the PD. D/EXK motif, in which the two carboxylates are responsible for Mg(2+) binding, the essential cofactor for the great majority of enzymes. The precise mechanism of cleavage has not yet been established for any enzyme, the main uncertainty concerns the number of Mg(2+) ions directly involved in cleavage. Cleavage in the two strands usually occurs in a concerted fashion and leads to inversion of configuration at the phosphorus. The products of the reaction are DNA fragments with a 3'-OH and a 5'-phosphate.

[Genetics and molecular medicine in cardiology]

The discoveries on molecular aspects of cellular function are changing the concepts of health and disease. All medical fields, including cardiology, have been enriched with several diagnostic test to determine predisposition and to detect molecular dysfunctions. This review on the genetic and molecular aspects of cardiovascular diseases is written at the Centenary of the rediscovery of Mendel's principles on heredity and at the time of the announcement of the end of the human genome sequencing task. The review starts with considerations on the pluricellular constitution of the human body, and the principles of genetics with their molecular bases; including a short description of the methods for gene mapping. The following sections give a historic synopsis on the concepts of medical genetics, molecular medicine, and the Human Genome Project. The review ends with a brief description of the spectrum of genetic diseases, using examples of cardiovascular diseases.

ATP-dependent restriction enzymes

The phenomenon of restriction and modification (R-M) was first observed in the course of studies on bacteriophages in the early 1950s. It was only in the 1960s that work of Arber and colleagues provided a molecular explanation for the host specificity. DNA restriction and modification enzymes are responsible for the host-specific barriers to interstrain and interspecies transfer of genetic information that have been observed in a variety of bacterial cell types. R-M systems comprise an endonuclease and a methyltransferase activity. They serve to protect bacterial cells against bacteriophage infection, because incoming foreign DNA is specifically cleaved by the restriction enzyme if it contains the recognition sequence of the endonuclease. The DNA is protected from cleavage by a specific methylation within the recognition sequence, which is introduced by the methyltransferase. Classic R-M systems are now divided into three types on the basis of enzyme complexity, cofactor requirements, and position of DNA cleavage, although new systems are being discovered that do not fit readily into this classification. This review concentrates on multisubunit, multifunctional ATP-dependent restriction enzymes. A growing number of these enzymes are being subjected to biochemical and genetic studies that, when combined with ongoing structural analyses, promise to provide detailed models for mechanisms of DNA recognition and catalysis. It is now clear that DNA cleavage by these enzymes involves highly unusual modes of interaction between the enzymes and their substrates. These unique features of mechanism pose exciting questions and in addition have led to the suggestion that these enzymes may have biological functions beyond that of restriction and modification. The purpose of this review is to describe the exciting developments in our understanding of how the ATP-dependent restriction enzymes recognize specific DNA sequences and cleave or modify DNA.

DNA restriction dependent on two recognition sites: activities of the SfiI restriction-modification system in Escherichia coli

In contrast to many type II restriction enzymes, dimeric proteins that cleave DNA at individual recognition sites 4-6 bp long, the SfiI endonuclease is a tetrameric protein that binds to two copies of an elongated sequence before cutting the DNA at both sites. The mode of action of the SfiI endonuclease thus seems more appropriate for DNA rearrangements than for restriction. To elucidate its biological function, strains of Escherichia coli expressing the SfiI restriction-modification system were transformed with plasmids carrying SfiI sites. The SfiI system often failed to restrict the survival of a plasmid with one SfiI site, but plasmids with two or more sites were restricted efficiently. Plasmids containing methylated SfI sites were not restricted. No rearrangements of the plasmids carrying SfiI sites were detected among the transformants. Hence, provided the target DNA contains at least two recognition sites, SfiI displays all of the hallmarks of a restriction-modification system as opposed to a recombination system in E. coli cells. The properties of the system in vivo match those of the enzyme in vitro. For both restriction in vivo and DNA cleavage in vitro, SfiI operates best with two recognition sites on the same DNA.

Short-range and long-range context effects on coliphage T4 endonuclease II-dependent restriction

Synthetic sites inserted into a plasmid were used to analyze the sequence requirements for in vivo DNA cleavage dependent on bacteriophage T4 endonuclease II. A 16-bp variable sequence surrounding the cleavage site was sufficient for cleavage, although context both within and around this sequence influenced cleavage efficiency. The most efficiently cleaved sites matched the sequence CGRCCGCNTTGGCNGC, in which the strongly conserved bases to the left were essential for cleavage. The less-conserved bases in the center and in the right half determined cleavage efficiency in a manner not directly correlated with the apparent base preference at each position; a sequence carrying, in each of the 16 positions, the base most preferred in natural sites in pBR322 was cleaved infrequently. This, along with the effects of substitutions at one or two of the less-conserved positions, suggests that several combinations of bases can fulfill the requirements for recognition of the right part of this sequence. The replacements that improve cleavage frequency are predicted to influence helical twist and roll, suggesting that recognition of sequence-dependent DNA structure and recognition of specific bases are both important. Upon introduction of a synthetic site, cleavage at natural sites within 800 to 1,500 bp from the synthetic site was significantly reduced. This suggests that the enzyme may engage more DNA than its cleavage site and cleaves the best site within this region. Cleavage frequency at sites which do not conform closely to the consensus is, therefore, highly context dependent. Models and possible biological implications of these findings are discussed.

How the EcoRI endonuclease recognizes and cleaves DNA

One popular recombinant DNA tool is the EcoRI endonuclease, which cleaves DNA at GAATTC sites and serves as a paradigm for sequence specific DNA-enzyme interactions. The recently revised X-ray crystal structure of an EcoRI-DNA complex reveals EcoRI employs novel DNA recognition motifs, a four alpha-helix bundle and two extended chains, which project into the major groove to contact substrate purines and pyrimidines. Interestingly, pyrimidine contacts had been predicted based on genetic and biochemical studies. Current work focuses on the EcoRI active site structure, enzyme and substrate conformational changes during catalysis, and host-restriction system interactions.

Base methylation and local DNA helix stability. Effect on the kinetics of cruciform extrusion

We have studied the effect of base methylation on the rate of cruciform extrusion. A number of inverted repeats with central restriction sites were methylated at N6-adenine and C-5-cytosine, and rate constants for cruciform extrusion at 37 degrees C were measured. The effect of A-methylation at two bases was to enhance the rate for extrusion by nearly fourfold, while C-methylation lead to reduced extrusion rates, by factors of 1.7 and 2.7. The bkb inverted repeat, which has a central GGATCC sequence, was independently and simultaneously methylated at adenine and cytosine. It was found that the effects of the two kinds of modification could be treated effectively independently. The results reveal the local helical destabilization and stabilization due to A and C-methylation, respectively.

Diagnosis of human endocrine disorders using recombinant DNA techniques

In this chapter we have reviewed several present and potential examples of DNA studies of hereditary endocrine disorders of humans. For the former, recombinant DNA studies have provided insights into the location and types of molecular derangements underlying these diseases. The gene alterations detected have in turn explained the aetiology of quantitative or qualitative alterations in the hormone product. The same methods used in these studies should be applicable to determining the aetiology of many other genetic disorders that affect these, as well as other, hormones for which specific DNA probes are or will become available.

A simple and rapid method for screening bacteria for type II restriction endonucleases: enzymes in Aphanothece halophytica

A method is described which allows a large number of bacterial strains to be rapidly and easily screened for the presence of site-specific endonucleases. The method involves selective permeabilization of the bacterial cell and analysis of the exuded material. Type II restriction endonucleases from cyanobacteria and Gram-negative eubacteria have been detected and new enzymes have been found. The method should be widely applicable and easy to modify for use in genera other than those tested. Three-site-specific endonuclease activities, detected by this method in Aphanothece halophytica PCC 7412, were purified and their recognition and cleavage specificities were determined AhaI and AhaII recognise and cleave the same DNA sequences as CauII and AcyI respectively; the specificity of AhaIII (TTT decreases AAA) has been reported previously (Whitehead and Brown, 1982, FEBS Letters 143:296-300).

The Canadian Rutherford lecture. An evolutionary view of disease in man

It is said that genes propose and experiences dispose. Biological adaptation (to fit with the experiences of life) implies functional and structural homeostasis. Disadaptation (the undoing of fitness, that is, morbidity, disease) occurs when experience overwhelms homeostasis or phenotypic variation undermines it. Human disease has social and cultural contexts, but it is also measurable in the biological dimensions of viability, development, reproduction, and longevity. Heritability is a description of ‘cause’ and, for some classes of disease, heritability is high in modern society relative to the past. Mendelian variation in man is immense; 90% of that occurring in single-copy DNA and associated with disease (selective mutation) is expressed in pre-reproductive life in the universal environment. Neutral (non-selective) mutations also occur in both coding and non-coding DNA regions; when the latter are tightly linked with selective mutations they can be used as markers of risk for disease. Accordingly, molecular genetics and other methods for defining genotype provide ways to anticipate risks to health and to prevent disease. To See a World in a Grain of Sand And a Heaven in a Wild Flower Hold Infinity in the palm of your hand And Eternity in an hour

Parasites at the origin of life

This paper is concerned with parasitic virus-like particles and their hosts. It is proposed that parasitism must have occurred at an early stage of evolution, soon after the first self-reproducing systems had formed. When chemical building blocks for self-reproducing systems became scarce, current theories envision that some self-reproducing systems evolved the capability to synthesize materials for self-replication from chemical precursors in the environment. It is proposed that at about the same time parasitic systems (phages) arose that replicated at the expense of host systems by diverting host materials to the replication of their own genomes. With the aid of a mathematical model we demonstrate that host and phages can coexist in a stable equilibrium, depending upon the carrying capacity of the environment. If the latter falls below a threshold, then the parasites die out. A parasite that has the capability to integrate into the host genome is replicated along with it and thus escapes extinction during periods of population bottlenecks of the host population. The presence of phages creates evolutionary pressures favoring host defenses against them. Thus, modern bacteria are able to degrade most invading DNA (through restriction enzymes). Defense capabilities require a share of the genome, thus adding to the genetic complexity of organisms.

Immunologic aspects of heart disease

All forms of heart disease, whether caused by streptococcus group A, aseptic heart injury, virus, hypersensitivity, autoimmune conditions, or graft-versus-host reactions, have in common allogenic transformation of the myocardial cell membrane and production of multifunctional autoantibodies, in addition to cause-specific antibodies, some of which cross-react with heart tissue. The outcome of this immunologic insult depends on the ability of the host's immunoregulatory mechanisms to dispose swiftly of the offending antigen and antibody or their complexes. Heart disease often results when these mechanisms, exemplified here, are not intact or when they function inappropriately in genetic or acquired settings and in varying haplotype frames.

Accumulation of spliced avian retrovirus mRNA is inhibited in S-adenosylmethionine-depleted chicken embryo fibroblasts

The synthesis and processing of B77 avian sarcoma virus RNA in infected chicken embryo fibroblasts was followed in the presence and absence of cycloleucine, a competitive inhibitor of the synthesis of S-adenosylmethionine and thus an inhibitor of RNA methylations. An increase in the steady-state levels of genome-length RNA and a decrease in the steady-state levels of subgenomic RNA molecules were obtained in the S-adenosylmethionine-depleted avian sarcoma virus-infected cells after 24 h of treatment with the inhibitor. The total number of virus-specific RNA molecules per cell, however, remained relatively constant under either condition. The production of newly synthesized virus-specific RNA in cycloleucine-treated and untreated cells infected with a transformation-defective strain of B77 avian sarcoma virus was followed as a function of [(3)H]uridine labeling time. The accumulation of radioactive genome-length 8.4-kilobase (kb) RNA continued in cycloleucine-treated cells, and virus particle production proceeded at normal rates as previously shown by incorporation of labeled nucleoside precursors or amino acids. In contrast, newly synthesized 3.5-kb subgenomic mRNA, the putative mRNA for the envelope protein precursor, failed to accumulate in the treated cells. The extent of the inhibition in the appearance of the radioactive 3.5-kb RNA was correlated with the extent of the inhibition of viral genomic and cellular mRNA methylations and was a function of the cycloleucine concentration. Under conditions in which the accumulation of 3.5-kb envelope protein mRNA was blocked by the cycloleucine treatment, there were significant increases in the rate of synthesis of the polypeptide products of the genome-length RNA, the precursors to the non-glycosylated gag proteins (Pr76(gag)), and the reverse transcriptase (Pr 180(gag pol)) relative to the rate of synthesis of the envelope protein precursor (gPr 92(env)). These results suggest that there is an S-adenosylmethionine requirement for the splicing, but not for the synthesis, packaging, or messenger function, of avian retrovirus genome-length RNA. Possible reasons for this requirement are discussed.

Pre-adipocyte determination either by insulin or by 5-azacytidine

CHEF/18 is a diploid Chinese hamster cell line of embryonic origin, which is fibroblastic in structure, but behaves like a mesenchymal stem cell line in its ability to differentiate into adipocytes, myoblasts, and chondrocytes. With these cells, adipocyte formation has been divided experimentally into two stages: (i) determination of pre-adipocytes, which have lost the ability to form other cell types while retaining their fibroblast structure; and (ii) commitment or terminal differentiation, in which lipids accumulate, adipocyte structure develops, and cells lose the ability to divide. This paper reports that the first stage can be induced by exposure to 5-azacytidine or 2'-deoxy-5-azacytidine, drugs that also induce CHEF cells to form other mesenchymal cell types, or by growth with added insulin. Pre-adipocytes are distinguished from CHEF stem cells by (i) their inability to form other mesenchymal cell types; and (ii) their rapid accumulation of lipid in response to added insulin. The possibility is discussed that both insulin and the cytidine analogs promote differentiation by the same mechanism, namely changes in the pattern of DNA methylation.

A tentative molecular-biological hypothesis for arteriosclerosis

In view of the fact that complications of arteriosclerosis are the most frequent causes of death in industrialized societies, its etiology is of enormous interest. The widely held lipid theory (detrimental effects of cholesterol) has been attacked because it cannot account for such facts as the homeostatic relationship of endogenous and exogenous cholesterol: for the "normal" cholesterol content of the early atheroma; for the distribution of the lesions, their spotty occurrence and their "programmed" appearance. Arteriosclerosis is part of the normal processes of aging which are related to molecular-biological changes. Autoimmune processes and the effects of extrachromosomal organisms of the genome (viruses, plasmids, viroids) are clinically of interest. Arteriosclerotic lesions are probably influenced by autoimmune processes; the variability and specificity of the non-chromosomal organisms may explain the location of the lesions; the end of the incubation period of the organisms may be responsible for the programmed appearance of clinical symptoms. The lipid changes are probably part of the adaptive mechanisms counteracting the rapid destruction of the vessels following the DNA alterations. Arteriosclerosis is part of normal evolution.

5-Methylcytosine in eukaryotic DNA

A small portion of the cytosine residues in the DNA of higher eukaryotes as well as in that of many lowe eukaryotes if methylated. The resulting 5-methylcytosine residues occur in specific in the DNA, usually adjacent to guanine residues on the 3' side. This methylation of eukaryotic DNA has been proposed to function in many ways, including control of transcription, maintenance of chromosome structure, repair of DNA, establishment of preferred sites for mutation, oncogenic transformation, and, in certain systems, protection of DNA against enzymatic degradation.