Can plant transposable elements geenrate novel regulatory systems?

Chromosome rearrangements and transposable elements

There has been limited corroboration to date for McClintock's vision of gene regulation by transposable elements (TEs), although her proposition on the origin of species by TE-induced complex chromosome reorganizations in combination with gene mutations, i.e., the involvement of both factors in relatively sudden formations of species in many plant and animal genera, has been more promising. Moreover, resolution is in sight for several seemingly contradictory phenomena such as the endless reshuffling of chromosome structures and gene sequences versus synteny and the constancy of living fossils (or stasis in general). Recent wide-ranging investigations have confirmed and enlarged the number of earlier cases of TE target site selection (hot spots for TE integration), implying preestablished rather than accidental chromosome rearrangements for nonhomologous recombination of host DNA. The possibility of a partly predetermined generation of biodiversity and new species is discussed. The views of several leading transposon experts on the rather abrupt origin of new species have not been synthesized into the macroevolutionary theory of the punctuated equilibrium school of paleontology inferred from thoroughly consistent features of the fossil record.

Plant transposable elements and the genome

Transposable elements are ubiquitous in the plant kingdom and share many common features, both structural and mechanistic, with mobile elements from other eukaryotes. Transposition of these elements can influence plant genes and genomes in many ways. It is also becoming clear that transposable element derived sequences can be a major component of plant genomes. These sequences are probably, therefore, very significant factors in plant evolution.

Mapping of the bovine oxytocin gene control region: identification of binding sites for luteal nuclear proteins in the 5' non-coding region of the gene

Abstract In view of the small number of hormone-producing cells, the factors regulating oxytocin gene expression in the classic site of synthesis, in the magnocellular neurons of the hypothalamus, have not yet been characterized. In the early bovine corpus luteum there is a tissue-specific oxytocin expression involving many more cells. This tissue therefore was chosen as a experimental system to identify deoxyribonucleic acid elements and nuclear proteins involved in the regulation of oxytocin gene expression. 3.2 kb from the 5'non-coding region of the bovine oxytocin gene have been sequenced and subcloned fragments used as probes for gel retardation and footprinting experiments. Binding sites for luteal as well as more ubiquitous proteins were detected in the oxytocin promoter region and in an artiodactyl-specific dispersed repeated deoxyribonucleic acid element. A binding site in the promoter region with a superficial similarity to an estrogen-responsive element (-159 to -152) was shown not to bind this steroid hormone receptor but to bind two nuclear proteins alternatively. One is a luteal protein, the other a more general transcription factor belonging to the steroid hormone receptor superfamily and similar, if not identical to the COUP protein. This alternative binding of a tissue- and phase-specifically expressed protein or an ubiquitous factor to the same site in the oxytocin promoter suggests a role for these two proteins in the transient up-regulation and subsequent down-regulation of the oxytocin gene during the differentiation of the bovine corpus luteum.

DNA sequence comparison of micropia transposable elements from Drosophila hydei and Drosophila melanogaster

Members of the retrotransposon family micropia were discovered as constituents of wild-type Y chromosomal fertility genes from Drosophila hydei. Several members of the micropia family have subsequently been recovered from Drosophila melanogaster and four micropia elements, micropia-DhMiF2, -DhMiF8, -Dm11 and -Dm2, two each from D. hydei and D. melanogaster, have been totally sequenced (17 kb of micropia sequences and 6.8 kb from insertions). Comparative analysis of micropia sequences revealed a complex pattern of divergence within a single Drosophila genome. The divergence includes deletions, possibly by a slipped mispairing mechanism, insertions of a retroposon, and of another retrotransposon (copia) and "positional nucleotide shuffling" within the tandem repeats of the 3' non-protein-coding region of micropia elements. A 10 bp long sequence of each repeat unit of the 3' tandem repeats of micropia elements is highly conserved and is therefore a candidate of functional importance either in transposition events or in regulatory activity on flanking DNA sequences.

An abundant testis RNA species shows sequence similarity to Y chromosomal and other genomic sites in Drosophila hydei

A cDNA clone bank was constructed from testis poly(A)+ RNA of Drosophila hydei and screened for clones which hybridize to Y chromosomal DNA sequences. The insert of clone cDhT14 hybridizes to a family of repeated DNA sequences with members distributed within the Y chromosome and elsewhere in the genome. This type of sequence has earlier been described as the Y-associated class of DNA. Southern blot analysis of DNA from different wild-type strains of D. hydei suggests that members of the T14 family of repeated DNA sequences are parts of a family of transposable elements. The genomic localization of the T14 family of repeated DNA sequences was revealed by in situ hybridization to metaphase and polytene chromosomes, and to transcripts of Y chromosomal lampbrush loops. Approximately 10-15 members (20%-30%) of the T14 sequence family reside in 8.3 kb PstI restriction fragments. A genomic clone of one of these DNA fragments, DhT14-8.3, hybridizes to transcripts on the Y chromosomal lampbrush loop "cones", and in conventional in situ hybridization experiments to region 12D/13A of the X chromosome and to region 112 of chromosome 5. The cDNA clone cDhT14 represents a part of an abundant testis RNA species of 5.0 kb. This RNA is also present in ovaries and in 0-3 h, 3-6 h and 6-12 h embryos, but less abundantly than in testes. Both the Y chromosomal site of the 8.3 kb PstI fragments and sites elsewhere in the genome are actively transcribed. At least one of the latter genomic sites is transcribed into the 5.0 kb RNA species.(ABSTRACT TRUNCATED AT 250 WORDS)

Retrotransposon-like sequences are expressed in Y chromosomal lampbrush loops of Drosophila hydei

The DNA sequence family micropia consists of repeated DNA sequences that occur dispersed in the genome of Drosophila hydei. Members of this DNA sequence family were recovered from two recombinant DNA clone banks obtained by microdissection of the two Y chromosomal lampbrush loop threads and pseudonucleolus from primary spermatocyte nuclei. Nucleotide sequence analysis of two of the recombinant DNA clones revealed homology to the DNA region coding for a reverse transcriptase-like protein in retroviruses and retrotransposons. Homologous tissue-specific transcripts of a size of 1.2 x 10(3) base-pairs were found in testes. Transcript in-situ hybridization shows that at least parts of these transcripts are synthesized in these Y chromosomal lampbrush loops, which were originally used for microdissection. Also the cytoplasm of primary spermatocytes contains homologous RNA species. These observations are discussed in the context of lampbrush loop function and evolution.