Elimination of Tec elements involves a novel excision process.

Abstract

Approximately 60,000 transposon-like elements of the Tecl and Tec2 families excise en masse from the micronuclear genome during formation of a macronucleus in Euplotes crassus. The circular product has been shown previously to contain the element inverted repeats joined head to head. To elucidate the mechanism of Tec excision, we have further characterized the circular products. DNA sequence analysis of cloned inverted repeat junctions and of a population of supercoiled Tec circles shows that the inverted repeat junctions consist of both copies of the target site duplication surrounding 10 additional bases. The 10 bp differs for each junction. We demonstrate that the circles are highly sensitive to Sl, mung bean and Bal 31 nucleases, and the site of sensitivity maps to the junction. Alkaline gel electrophoresis indicates that the junction does not contain a nick or gap; thus, a likely explanation for the nuclease sensitivity is the existence of a heteroduplex DNA structure at the junction. On the basis of these results, we present a model of Tec excision and discuss the relationship of Tec excision to IES elimination and chromosome fragmentation in E. crassus.

Epigenetic self-regulation of developmental excision of an internal eliminated sequence on Paramecium tetraurelia.

“…High-copy transposon-like elements identified in the germ-line genome of several other ciliates are recognizable members of this family (Doak et al 1994). In Euplotes crassus, developmental excision of both transposon-like elements and single-copy IESs generates free circular forms with an unusual heteroduplex covalent junction (Jaraczewski and Jahn 1993;Klobutcher et al 1993).…”

“…Can the model still apply to the production of the IES + macronuclear version? If excision is initiated by a cut at each end of the IES, as proposed in hypotrichous ciliates for both IESs and transposon-like elements (Klobutcher et al 1993;Jaraczewski and Jahn 1993;Williams et al 1993), and shown for a programmed deletion event in the more closely related Tetrahymena (Saveliev and Cox 1995), a prediction of the model is that retention of the G-gene IES in the new macronucleus will critically depend on the physical linkage of IES and flanking sequences in the old macronucleus. This is not borne out by the transformation of IES-cells with plasmid pIES, which results in IES retention.…”

DNA computing in vitro and in vivo

“…Existing data suggest that the presence of direct repeats alone is not enough to guide the unscrambling process. Instructions for unscrambling the genes are seemingly carried by the gene itself [21] and they might be either structural "instructions" or control factors like the presence of For example, in Tetrahymena and E. crassus species, within the micronuclear DNA, the two ends of the sequence to be eliminated are immediately flanked by a specific 5-mer sequence which is a crucial element of recognition [9,21].…”

Extreme heterogeneous composition of the Paramecium caudatum macronuclear genomic DNA between hemoglobin and nucleosome assembly protein-1 genes

“…Upon excision of the TA-IESs, one of the 5'-TA-3' direct repeats remains in the macronucleus-destined sequences (Steele et al, 1994). Sequencing of many IESs also revealed that the inverted repeat (IR) consensus sequences from Euplotes crassus (5'-TATAGAGG-3', Jaraczewski and Jahn, 1993) and P. aurelia complex (5'-TAYAGYNR-3', Klobutcher and Herrick, 1995) were similar to those of the metazoan Tc1/mariner transposons (Klobutcher and Herrick, 1995). This suggested that the origin of ciliate IESs might have been some type of transposons (Klobutcher and Jahn, 1991;Doak et al, 1994).…”