The lack of plastidal transit sequence cannot override the targeting capacity of Bradyrhizobium japonicum δ-aminolevulinic acid synthase in transgenic rice

Abstract

The δ-aminolevulinic acid synthase (ALA-S) is an enzyme which catalyzes the synthesis of δ-aminolevulinic acid (ALA). The Bradyrhizobium japonicum ALA-S coding sequence lacking plastidal transit sequence was introduced into the rice genome (C line). The transgenic lines, C4 and C5, were compared with the transgenic lines expressing TALA-S gene with plastidal transit sequence (P line) to investigate whether the plastidal sequence affects the targeting capacity of B. japonicum ALA-S gene and the ALA-synthesizing capacity in rice plants. The B. japonicum ALA-S mRNA was expressed efficiently in C lines and the protein was localized in the stroma of chloroplasts regardless of the transit sequence as in P lines. The resulting transgenic plants, C line, had similar levels of ALA-S activity, ALA, protoporphyrin IX and chlorophylls, compared to those of P lines. In response to irradiance of 350 μmol m⁻² s⁻¹, transgenic lines C4 and C5 displayed the characteristic phenotypes of photodynamic damage, i.e., decreases in photosynthetic parameter F_v/F_m, as in P5 and P14 lines, whereas wild type did not. These results indicate that the lack of the plastidal transit sequence influences neither chloroplast translocation of B. japonicum ALA-S nor ALA-synthesizing capacity in the transgenic rice.