Helitrons: Enigmatic abductors and mobilizers of host genome sequences

Helitrons are a recently discovered superfamily of eukaryotic transposable elements. They are known for their ability to capture and mobilize gene fragments and, in turn, significantly contribute to the lack of gene colinearity widely reported among different maize inbred lines. As judged by present evidence, Helitrons differ fundamentally from Class I and Class II families of transposable elements in both structure and the mechanism of transposition. These elements are poorly understood despite their massive abundance, their structural diversity and the important role they apparently play in the evolution of maize genome. Although evidence for recent Helitron movement has been reported in maize, autonomous Helitron activity has not been reported. Helitrons are postulated to transpose via a so-called rolling circle (RC) mechanism involving strand replacement catalyzed by helicase and replicase enzymes encoded by an autonomous element. However, no experimental evidence in support of this hypothesis has been reported. Several models for gene piece capture have been proposed based on the structural features of Helitrons and the comparison of captured gene fragments of related elements. However no supporting evidence is extant for any of the models proposed. A better understanding of these elements requires concrete evidence of their transposition in the present day genome, establishment of a system to assay their transposition and an analysis of additional indigenous Helitrons in other species. This review critically analyses the proposed mechanisms of Helitron transposition, their impact on genome evolution and the process by which these enigmatic elements capture and multiply host genes.