Laten, H.M., E.R. Havecker2, L.M. Farmer1 & D.F. Voytas1. 2002. Endogenous retroviruses as transgenic vectors - the ubiquitous SIRE1 family. †Ninth Biennial Conference on the Cellular and Molecular Biology of the Soybean, Aug 11-14, Champaign, IL †USA
1Biology, Loyola University Chicago†††††††††††††
2Zoology & Genetics, Iowa State University
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, lotus, favabean, Arabidopsis, tomato, and maize.† 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.† Readthrough assays with GUS constructs support this suggestion.
Ten 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 (<0.0009 for SIRE1-4 and SIRE1-7) have been used to date their insertions to less than 30,000 years ago. Two of the ten SIRE-1 copies are adjacent to novel LTR retrotransposons suggesting that, like Z. mays, some LTR retroelements may be clustered in the G. max genome.
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 long and possibly recent history of integrating into the genomes of monocots and dicots would be a welcome addition to the existing collection of gene transfer strategies.