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

Abstract 

Two gene constructs (pROK.TG1L and pROK.TG1LK) were utilized to achieve accumulation of maize γ-zein to high levels in tobacco (Nicotiana tabacum L.) leaves. Both the chimaeric genes contained the γ-zein-coding region preceded 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 γ-zein and γ-zein:KDEL in leaves was compared with heterologous protein accumulation in tobacco plants previously transformed with a γ-zein cDNA harbouring a native 5′UTR. Replacement of γ-zein 5′UTR with the TMV leader dramatically increased γ-zein production. Furthermore, γ-zein:KDEL-expressing plants, on average, accumulated twice as much foreign protein in their leaves as pROK.TG1L plants. The two-fold increase in the level of γ-zein:KDEL can probably be attributed to an improvement in the mechanism for ER retention of zeins in the transgenic cells. Transformants also showed increased production of BiP, though to a lesser extent in γ-zein:KDEL-expressing plants compared with pROK.TG1L plants. It is therefore likely that γ-zein:KDEL retention is made less dependent on the chaperone assistance of BiP by the presence of the KDEL signal on the γ-zein mutant.