Base compositions of the mRNA three-prime untranslated region (3′UTR), represented by the 50-base region that is 50 bases away from the polyadenylation [poly(A)] site^a.

a:Poly(A) sites were mapped using NCBI mRNA sequences. b:The mean U/A ratio is significantly higher in plants than in animals according to Student’s t-test (P = 0.0006, with a two tailed distribution and two-sample equal variance model).