A nest of long terminal repeat (LTR) retrotransposons (RTRs), discovered by LTR_STRUC analysis, is near core genes encoding the NPR1 disease resistance-activating factor and a
heat-shock-factor-(HSF-) like protein in sugarbeet hybrid US H20. SCHULTE, a 10 833 bp LTR retrotransposon, with 1372 bp LTRs that are 0.7% divergent, has two ORFs with unexpected introns but encoding a reverse transcriptase with rve and Rvt2 domains similar to Ty1/copia-type retrotransposons and a hypothetical protein. SCHULTE produced significant nucleotide BLAST alignments with repeat DNA elements from all four families of plants represented in the TIGR plant repeat database (PRD); the best nucleotide sequence alignment was to ToRTL1 in Lycopersicon esculentum. A second sugarbeet LTR retrotransposon, SCHMIDT, 11 565 bp in length, has 2561 bp LTRs that share 100% identity with each other and share 98-99% nucleotide sequence identity over 10% of their length with DRVs, a family of highly repetitive, relatively small DNA sequences that are widely dispersed over the sugarbeet genome. SCHMIDT encodes a complete gypsy-like polyprotein in a single ORF. Analysis using LTR_STRUC of an in silico deletion of both of the above two LTR retrotransposons found that SCHULTE and SCHMIDT had inserted within an older LTR retrotransposon, resulting in a nest that is only about 10 Kb upstream of NPR1 in sugarbeet hybrid US H20.
We describe discovery in Beta vulgaris L. of Coe1, a DNA transposase gene within putative long terminal repeats (LTRs), and other retrotransposon-like features including both a retroviral-like hypothetical gene and an Rvt2-domain reverse transcriptase pseudogene. The central DNA transposase gene encodes, in eight exons, a predicted 160-KDa protein producing BLAST alignments with En/Spm-type transposons. Except for a stop signal, another ORF encodes a Ty1-copia-like reverse transcriptase with amino acid sequence domain YVDDIIL. Outside apparent LTRs, an 8-mer nucleotide sequence motif CACTATAA, near or within inverted repeat sequences, is hypothetical extreme termini. A genome scan of Arabidopsis thaliana found another example of a Tnp2-domain transposase gene within an apparent LTR-retrotransposon on chromosome 4.
NPR1 is a gene of central importance in enabling plants to resist microbial attack. Therefore, knowledge of nearby genes is important for genome analysis and possibly for improving disease resistance. In this study, systematic DNA sequence analysis, gene annotation, and protein BLASTs were performed to determine genes near the NPR1 gene in Beta vulgaris L., Medicago truncatula Gaertn, and Populus trichocarpa Torr. & Gray, and to access predicted function. Microsynteny was discovered for NPR1 with genes CaMP, encoding a chloroplast-targeted signal calmodulin-binding protein, and CK1PK, a CK1-class protein kinase. Conserved microsynteny of NPR1, CaMP, and CK1PK in three diverse species of eudicots suggests maintenance during evolution by positive selection for close proximity. Perhaps close physical linkage contributes to coordinated expression of these particular genes that may control critically important processes including nuclear events and signal transduction.
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