Transposon expression and potential effects on gene regulation of Brassica rapa and B. oleracea genomes

Transposons or transposable elements (TEs) are ubiquitous and most abundant DNA components in higher eukaryotes.Recent sequencing of the Brassica rapa and B. oleracea genomes revealed that the amplification of TEs is oneof the main factors inducing the difference in genome size.However, the expressions of TEs and the TE effects on gene regulation and functions of these two Brassica diploid species were unclear.Here, we analyzed the RNA sequencing data of leaves, roots, and stems from B. rapa and B. oleracea.Our data showed that overall TEs in either genome expressed at very low levels, and the expression levels of different TE categories and families varied among different organs.Moreover, even for the same TE category or family, the expression activities were distinct between the two Brassica diploids.Forty-one and nine LTR retrotransposons with the transcripts that read into their adjacent sequences have the distances shorter than 2 kb and 100 bp compared to the downstream genes.These LTR retrotransposon readout transcriptions may produce sense or antisense transcripts of nearby genes, with the effects on activating or silencing corresponding genes.Meanwhile, intactLTRs were detected at stronger readout activities than solo LTRs.Of the TEs inserted into genes, the frequencies were observed at a higher level in B. rapa than in B. oleracea.In addition, DNA transposons were prone to insert or retain in the intronic regions of genes in either Brassica genomes.These results revealed that the TEs may have potential effects on regulating protein coding genes.