Plant endogenous retroviruses --Poised for transformation?

Howard M. Laten1, Ericka R. Havecker2, Lisa M. Farmer1 and Daniel F. Voytas2

1Department of Biology, Loyola University Chicago, Chicago, IL 60626; 2Department of Zoology & Genetics, Iowa State University, Ames, IA 50011

Topic Area: Transformation Technology

SIRE1 is a family of soybean retroelements that in addition to gag, protease, integrase and reverse transcriptase domains - the standard features of retrotransposons - encodes an envelope-like protein.Homologs of SIRE1 have been detected in Glycine soja, other Glycine species, Arabidopsis, maize, and tomato.Unlike most plant retroelements, members of the SIRE1 family contain intact ORFís with full coding capacity.A single stop codon separates the gag-pol ORF from the env ORF.The possibility of a readthrough mechanism is supported by a conserved stop-codon context which matches a consensus sequence (UAGCARYYA) found in several ssRNA plant viruses that is required for translational readthrough.

Eight members of the family have been sequenced.These members are remarkably homogeneous.An analysis of base-pair substitutions between members supports the inference that the small degree of divergence has occurred under selection.In addition, since the LTRís of each element are identical at the time of integration, the divergences between pairs of LTRís (0.05% for SIRE1-4 and SIRE1-9) have been used to date their insertions (as young as 24,000 years).

SIRE1 has successfully colonized the genomes of a wide variety of plants, and the sequenced copies from G. max appear to be functional and of recent origin.The presence of an envelope-like ORF strongly suggests that SIRE1 is an endogenous retrovirus capable of infectious transfer.Selective maintenance of the envelope ORF suggests that the protein it encodes is functional.†† Harnessing these qualities for the development of novel transformation vectors is underway.An infectious agent that has a history of integrating into the genomes of monocots and dicots would be a welcome addition to the existing collection of gene transfer strategies.