Strong Expression and Conserved Regulation of ACT2 in Arabidopsis thaliana and Physcomitrella patens

Arabidopsis ACT2 represents an ancient class of vegetative plant actins and is strongly and constitutively expressed in almost all Arabidopsis sporophyte vegetative tissues. Using the beta glucuronidase report system, the studies showed that ACT2 5' regulatory region was significantly more active than CaMV 35S promoter in Arabidopsis seedlings and gametophyte vegetative tissues of Physcomitrella patens. Its activity was also observed in rice and maize seedlings. Thus, the ACT2 5' regulatory region could potentially serve as a strong regulator to express a transgene in divergent plant species. ACT2 5' regulatory region contained 15 conserved sequence elements, an ancient intron in its 5' un-translated region (5' UTR), and a purine-rich stretch followed by a pyrimidine-rich stretch (PuPy). Mutagenesis and deletion analysis illustrated that some of the conserved sequence elements and the region containing PuPy sequences played regulatory roles in Arabidopsis. Interestingly, mutation of the conserved elements did not lead a dramatic change in the activity of ACT2 5' regulatory region. The ancient intron in ACT2 5' UTR was required for its strong expression in both Arabidopsis and P. patens, but did not fully function as a canonical intron. Thus, it was likely that some of the conserved sequence elements and gene structures had been preserved in ACT2 5' regulatory region over the course of land plant evolution partly due to their functional importance. The studies provided additional evidences that identification of evolutionarily conserved features in non-coding region might be used as an efficient strategy to predict gene regulatory elements.