POTENTIAL RETROVIRUSES IN PLANTS - TAT1 IS RELATED TO A GROUP OF ARABIDOPSIS-THALIANA TY3 GYPSY RETROTRANSPOSONS THAT ENCODE ENVELOPE-LIKE PROTEINS/

Tat1 was originally identified as an insertion near the Arabidopsis th aliana SAM1 gene. We provide evidence that Tat1 is a retrotransposon a nd that previously described insertions are solo long terminal repeats (LTRs) left behind after the deletion of coding regions of full-lengt h elements. Three Tat1 insertions were characterized that have retrotr ansposon features, including a primer binding site complementary to an A. thaliana asparagine tRNA and an open reading frame (ORF) with simi lar to 44% amino acid sequence similarity to the gag protein of the Ze a mays retrotransposon Zeon-1. Tat1 elements have large, polymorphic 3 ' noncoding regions that may contain transduced DNA sequences; a 477-b ase insertion in the 3' noncoding region of the Tat1-3 element contain s part of a related retrotransposon and sequences similar to the nontr anslated leader sequence of AT-P5C1, a gene for pyrroline-5-carboxylat e reductase. Analysis of DNA sequences generated by the A. thaliana ge nome project identified 10 families of Ty3/gypsy retrotransposons, whi ch share up to 51 and 62% amino-acid similarity to the ORFs of Tat1 an d the A. thaliana Athila element, respectively. Phylogenetic analyses resolved the plant Ty3/gypsy elements into two lineages, one of which includes homologs of Tat1 and Athila. Four families of A. thaliana ele ments within the Tat/Athila lineage encode a conserved ORF after integ rase at a position occupied by the envelope gene in retroviruses and i n some insect Ty3/gypsy retrotransposons. Like retroviral envelope gen es, this ORF encodes a transmembrane domain and, in some insertions, a putative secretory signal sequence. This suggests that Tat/Athila ret rotransposons may produce enveloped virions and may be infectious.