Accumulation of maize gamma-zein and gamma-zein: KDEL to high levels in tobacco leaves and differential increase of BiP synthesis in transformants

gene constructs (pROK.TG1L and pROK.TG1LK) were utilized to achieve accumul ation of maize gamma -zein to high levels in tobacco (Nicotiana tabacum L.) leaves. Both the chimaeric genes contained the gamma -zein-coding region p receded by the 5'untranslated leader from the coat protein mRNA of TMV, but one of them (pROK.TG1LK) was modified in its protein-coding region by the addition of the ER retention signal KDEL. The accumulation of gamma -zein a nd gamma -zein:KDEL in leaves was compared with heterologous protein accumu lation in tobacco plants previously transformed with a gamma -zein cDNA har bouring a native 5'UTR. Replacement of gamma -zein 5'UTR with the TMV leade r dramatically increased gamma -zein production. Furthermore, gamma -zein: KDEL-expressing plants, on average, accumulated twice as much foreign prote in in their leaves as pROK.TG1L plants. The twofold increase in the level o f gamma -zein:KDEL can probably be attributed to an improvement in the mech anism for ER retention of zeins in the transgenic cells. Transformants also showed increased production of BiP, though to a lesser extent in gamma -ze in:KDEL-expressing plants compared with pROK.TG1L plants. it is therefore l ikely that gamma -zein:KDEL retention is made less dependent on the chapero ne assistance of BiP by the presence of the KDEL signal on the gamma -zein mutant.