BNR - a LINE family from Beta vulgaris - contains a RRM domain in open reading frame 1 and defines a L1 sub-clade present in diverse plant genomes.

We characterized a novel type of plant non-LTR retrotransposons, identified as the BNR family, in sugar beet (Beta vulgaris) genomes. Although their ORF2 sequences were similar to those of previously analysed LINEs (long interspersed nuclear elements) of the L1 clade, their ORF1 sequences differ strongly from those of most plant LINEs. Two novel domains were identified, containing a conserved secondary motif, known as the RNA recognition motif (RRM). ORF1 lacks the zinc finger motif that is typical of plant LINEs, but has an RRM that is likely to have a RNA-binding function. BNR LINEs are highly diverse, and were characterized by gel-blot and fluorescent in situ hybridization, showing a widespread occurrence and clustering along chromosome arms. Insertion of BNR1 into a well-described satellite repeat was detected in two cultivars only, indicating recent activity. Database searches revealed the existence of LINE families possessing an ORF1 sequence similar to that of BNR in the genomes of higher plants such as poplar, lotus and soybean. Comparing their reverse transcriptase regions with those of other retrotransposons, these LINEs were assigned to the L1 clade, but form a distinct group, providing evidence of a major separation of L1 elements in plants. This indicates a common origin of BNR-like LINEs, suggesting that these elements form a sub-clade designated as the BNR sub-clade.