A new class of LINEs (ATLN-L) from Arabidopsis thaliana with extraordinary structural features.
The Arabidopsis thaliana genome has about 250 copies of LINEs (here called ATLNs). Of these, some, called ATLN-Ls, have an extra sequence of about 2 kb in the region downstream of two consecutive open reading frames, orf1 and orf2. Interestingly, the extra sequences in these ATLN-L members have another open reading frame, designated as orf3. Each member is flanked by direct repeats of a target site sequence, showing that ATLN-L members with the three open reading frames have retrotransposed as a unit. The ATLN-L members are also distinct from other ATLN members: orf1 terminates with TAA (or TAG) and is located in the same frame as orf2, and the ATG initiation codon of orf2 is not present in the proximal region. A sequence that may form a pseudoknot structure in ATLN-L mRNA was present in the proximal region of orf2, therefore the TAA (or TAG) termination codon of orf1 is assumed to be suppressed to produce an Orf1-Orf2 transframe protein during the translation of the ATLN-L mRNA. The region between orf2 and orf3 is several hundred bp long, suggesting that orf3 expression is independent of orfl-orf2. The amino acid sequences of the proteins Orf1 and Orf3 are highly homologous in their N-terminal half regions that have a retroviral zinc-finger motif for RNA binding. Orf3, however, has a leucine-zipper motif in addition to the zinc-finger motif. The C-terminal regions of the Orf1 and Orf3 proteins have poor homology, but seem to have nuclear localization signals, suggesting that these proteins are involved in the transfer of ATLN-L mRNA to nuclei. A phylogenetic tree shows that Orf3 proteins form a branch distinct from the branches of the Orf1 proteins encoded by ATLN-L members. This indicates that an ancestor element of ATLN-Ls has incorporated the orf1 frame carried by another ATLN member into its distal region to orf1-orf2 during evolution.
Noma
K
K
Institute of Molecular and Cellular Biosciences, The University of Tokyo, Japan.
Ohtsubo
H
H
Ohtsubo
E
E
D020121:5' Untranslated Regions / Q000235:genetics; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000235:genetics*; D001483:Base Sequence; D018744:DNA, Plant / Q000235:genetics; D020084:Long Interspersed Nucleotide Elements / Q000235:genetics*; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D010802:Phylogeny; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics; D018626:Retroelements; D017386:Sequence Homology, Amino Acid; D012689:Sequence Homology, Nucleic Acid
D020121:5' Untranslated Regions; D018744:DNA, Plant; D010940:Plant Proteins; D018626:Retroelements