Abstract:Three plant high expression vectors harboring 25, 50 and 100 deoxyadenylate (dA) residues respectively in 3‘ untranslated region (3‘-UTR) were constructed by inserting poly(dA) sequence into the primary vector containing CaMV 35S promoter doubled with region B and II which is a leader sequence derived from tobacco mosaic virus, within 5‘-UTR. Transient expression of chimeric GUS gene in transgenic tobacco (Nicotiana tabacum L. ) mesophyll protoplasts showed that:doubled enhancer, Ω and poly (dA) increasd GUS expression. When both Ω and poly (dA) were present, the level of expression increased further, compared to that when only Ω was present. Moreover, when Ω was present, doubling the length of poly (dA) resulted in a further increase in GUS expression, which suggested a positive relationship between poly(dA) length and the level of expression.