PCR amplification products are of limited use for the study of DNA/protein interaction

Genomic simple repetitive DNAs are targets for differential binding of nuclear proteins

The biological meaning of abundant simple repetitive DNA sequences in eukaryote genomes is obscure. Therefore, (GAA)n, (GT)n, and composite (GT)n(GA)m, blocks were characterized for protein binding in the repeat and flanking sequences of cloned genomic DNA fragments. In gel mobility shift and competition assays the binding of nuclear proteins to the repeats was specific (including some flanking single copy sequences). DNase footprinting revealed the target sequences within and adjacent to the repeats. Chemical modifications (OsO4, DEPC) demonstrated non-B DNA structures in the polypurine blocks. The binding of nuclear proteins in and around simple repeat sequences refute biological insignificance of all of these ubiquitously interspersed elements.

Simple repetitive sequences in the genome: structure and functional significance

The current explosion of DNA sequence information has generated increasing evidence for the claim that noncoding repetitive DNA sequences present within and around different genes could play an important role in genetic control processes, although the precise role and mechanism by which these sequences function are poorly understood. Several of the simple repetitive sequences which occur in a large number of loci throughout the human and other eukaryotic genomes satisfy the sequence criteria for forming non-B DNA structures in vitro. We have summarized some of the features of three different types of simple repeats that highlight the importance of repetitive DNA in the control of gene expression and chromatin organization. (i) (TG/CA)n repeats are widespread and conserved in many loci. These sequences are associated with nucleosomes of varying linker length and may play a role in chromatin organization. These Z-potential sequences can help absorb superhelical stress during transcription and aid in recombination. (ii) Human telomeric repeat (TTAGGG)n adopts a novel quadruplex structure and exhibits unusual chromatin organization. This unusual structural motif could explain chromosome pairing and stability. (iii) Intragenic amplification of (CTG)n/(CAG)n trinucleotide repeat, which is now known to be associated with several genetic disorders, could down-regulate gene expression in vivo. The overall implications of these findings vis-à-vis repetitive sequences in the genome are summarized.

Indirect DNA/gene diagnoses via electrophoresis--an obsolete principle?

In principle, gene defects can be investigated directly or indirectly via informative polymorphisms in their vicinity. But because many defects are not yet defined molecularly, these inherited diseases can only be diagnosed indirectly via analysis of informative family situations. Since (multiple) mutation analyses, e.g. via DNA sequencing, are time-consuming and expensive, indirect analysis may still be performed initially--particularly in diseases caused by heterogenous mutations. We focus on diagnoses of neurological and (auto)immune diseases by polymerase chain reaction and separation of the DNA fragments via gel electrophoreses. Even after gene defects have been identified, indirect analysis might be necessary, for example in Huntington's chorea. Although this genetic defect has been characterized as a trinucleotide disease, indirect DNA diagnosis is still performed in particular cases for psychological reasons. The causes of autoimmune diseases are multifactorial and the inheritance is complex, involving several genes. Genome-wide screening programs may involve indirect approaches via analyses of polymorphic microsatellites. Large parts of the immunological genome can be covered when 20 or more genes are investigated simultaneously. Thus the genetic bases of autoimmune diseases are disclosed. Microsatellites themselves could have a biological meaning. We therefore discuss also DNA/protein interactions for simple tandem repeats, the major targets for indirect gene diagnoses. Only indirect evidence exists that certain simple repeats influence genomic (in)stability. Taken together, indirect gene diagnoses supplement direct approaches in a variety of different purposes and in combination with standard electrophoresis techniques.

Altered electrophoretic behavior of DNA due to short-time UV-B irradiation

UV-B irradiation is often inevitable for visualization of DNA fragments after ethidium bromide staining. Three different simple-repeat-containing, double-stranded genomic DNA fragments were analyzed for UV-B (312 nm) damage using different gel electrophoretic systems. The effects of UV-B light were obvious after 5 min (31.5 kJ/m2) of irradiation in native polyacrylamide gel electrophoresis (PAGE). Standard single-strand conformation analyses revealed no alterations while a modification did. Sodium dodecyl sulfate (SDS)-PAGE was found to be highly sensitive with regard to the detection of damages and their time/dosage dependency. In addition, SDS-PAGE analysis pointed to different events occurring during UV-B irradiation. Alterations in DNA conformation were detected in every single strand analyzed after 1 min (6.3 kJ/m2) of UV-B exposure. Gel retardation analyses revealed significant changes of protein binding to target DNAs after 2 min of irradiation--possibly stemming from structural modifications and/or originating from binding sites for proteins involved in DNA repair.

The analysis of simple repeat loci as applied in evolutionary and behavioral sciences

This chapter describes several aspects of tandemly organized, simple repetitive DNA sequences and their usefulness for genetic relationship analyses. After introducing the structure, the evolution and the biological meaning of such target sequences in a particularly well-studied gene, we discuss oligonucleotide probes for generating individual specific multilocus banding patterns. Thus, oligonucleotide fingerprinting allows to approach novel problems in behavioral sciences. Here, we use a passerine bird, the great tit (Parus major) as an example. Finally, genomic fingerprinting is compared to sensitive amplification methods requiring less DNA. Advantages and shortcomings of these techniques need to be evaluated in the context of the biological question(s) asked and, above all, the quality and quantity of the starting material.

On the essence of "meaningless" simple repetitive DNA in eukaryote genomes

Various kinds of simple tandemly repetitive DNA sequences are abundantly interspersed in the genomes of practically all eukaryotic species studied. The comparatively elevated mutation rates of simple repeat blocks result in highly polymorphic and therefore extremely informative investigation systems for studies on forensic, ecological and genetic relationship questions. Recently the techniques for analyzing simple repeats have achieved great effectivity and simplicity. Beyond their utility as tools for differentiation and individualization, certain of these repeated elements harbor quite unexpected qualities which may be discussed in the context of their biological meaning. i) A specific subset of simple (cac)n or (gtg)n repeats is expressed in mature mRNA and total cellular RNA. ii) Despite the apparently high mutation rate certain (gt)n or mixed (gt)n/(ga)m stretches of intronic simple repeats are preserved in immunologically relevant genes for at least 70 x 10(6) years and they bind nuclear protein molecules with high affinities. Consequently in addition to their tool character, the biological aspects of simple repeated DNA should be taken into consideration.