Transposable elements (TEs) are mobile genetic entities ubiquitously distributed in nearly all genomes.High frequency of codons ending in A/T in TEs has been previously observed in some species.In this study,the biases in nucleotide composition and codon usage of TE transposases and host nuclear genes were investigated in the AT-rich genome of Arabidopsis thaliana and the GC-rich genome of Oryza sativa.Codons ending in A/T are more frequently used by TEs compared with their host nuclear genes.A remarkable positive correlation between highly expressed nuclear genes and C/G-ending codons were detected in O.sativa (r=0.944 and 0.839,respectively,P<0.0001) but not in A.thaliana,indicating a close association between the GC content and gene expression level in monocot species.In both species,TE codon usage biases are similar to that of weakly expressed genes.The expression and activity of TEs may be strictly controlled in plant genomes.Mutation bias and selection pressure have simultaneously acted on the TE evolution in A.thaliana and O.sativa.The consistently observed biases of nucleotide composition and codon usage of TEs may also provide a useful clue to accurately detect TE sequences in different species.
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