Structure of the Chlorella Zepp retrotransposon: nested Zepp clusters in the genome.

Zepp elements found in the telomeric region of Chlorella chromosomes show the characteristic features of non-viral (LINE-like) retrotransposons, including a poly(A) tail, 5' truncations, a retroviral reverse transcriptase-like ORF and flanking target duplications. We have isolated and characterized a full-length Zepp element (8943 bp long) from Chlorella chromosome V. Some peculiar features of this element, including nested integration, two ORF structures, a long 3' noncoding region and a possible promoter region are compared with those of the Drosophila telomeric retrotransposons HeT-A and TART. The Chlorella chromosome-Zepp system appears to represent an intermediate stage between canonical telomerase-telomeres and Drosophila retrotransposon-telomeres.