Processing of a chimeric protein in chloroplasts is different in transgenic maize and tobacco plants

Transgenic maize (Zea mays L.) and tobacco (Nicotiana tabacum Petit Havana SR1) plants have been generated, which overproduce a mitochondrial Nicotiana plumbaginifolia manganese superoxide dismutase (MnSOD) in chloroplasts. For this, the mature MnSOD-coding sequence was fused to a chloroplast transit peptide from a Pisum sativum ribulose-1,5-bisphosphate carboxylase (Rubisco) gene and expression of the chimeric gene was driven by the cauliflower mosaic virus (CaMV) 35S promoter. The transgenic MnSOD gene product was correctly targeted to the chloroplasts both in maize and tobacco. However, despite the use of the CaMV 35S promoter, the MnSOD was predominantly localized in the chloroplasts of the bundle sheath cells of maize. Furthermore, the transit peptide was cleaved off at a different position in maize and tobacco