The presence of a single rice prolamine or maize 10-kDa 8 - zein cis element results in localization of the RNA not only to the PB-ER but also to the cisternal ER. In the absence of any cis element, RNAs are localized to the cisternal ER. These results support the existence of a default RNA transport to the cis-ternal ER in addition to a regulated transport pathway to the PB-ER. RNA transport to the ER occurs even for RNAs that code for cytoplasmic-localized proteins (Hamada et al. 2003a), indicating that the bulk, if not all, of protein synthesis occurs near or on the ER.
Although glutelin RNAs are localized to the cisternal ER, their transport and targeting to this ER subdomain also occurs by a regulated pathway. Replacement of the prolamine or 10-kDa 8-zein 3'UTR with the glutelin 3'UTR results in their redirection from the PB-ER to the cisternal ER. These results suggest the presence of one or more cis elements in the glutelin 3'UTR, and the existence of a regulated glutelin pathway to the cisternal ER which is dominant over the regulated prolamine pathway to the PB-ER (Choi et al. 2000; Hamada et al. 2003b).
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