Tight translational control by the initiation factors eIF4E and eIF(iso)4E is required for maize seed germination

A characteristic mechanism of gene expression regulation during seed germination is the selective translation of mRNAs. Previous findings indicate that the two cap-binding complexes eIF4F (with eIF4E and eIF4G subunits) and eIF(iso)4F [with eIF(iso)4E and eIF(iso)4G subunits] are differentially expressed during maize seed germination. In addition, several studies in vitro have suggested that these factors may participate in selective mRNA translation. The translational activities of eIF4E and eIF(iso)4E were tested in vitro using transcripts from two different sets: dry (0 h) and 24-h-imbibed maize embryonic axes. In vitro translation of these mRNA pools in the presence of the recombinant eIF4E or eIF(iso)4E, and the native cap-binding complexes from dry-or 24-h-imbibed axes, produced different profiles of proteins which were visualized by two-dimensional protein gels and autoradiography. The data indicated that eIF(iso)4E was particularly required for translation of the stored mRNAs from dry seeds, and that eIF4E was unable to fully replace the eIF(iso)4E activity. In addition, the dry seed mRNA pool was translated by the cap-binding complex isolated from dry seeds better than by the complex isolated from 24-h-imbibed seeds, whereas the translational efficiency of the mRNA pool from 24-h-imbibed seeds was similar between the cap-inding complexes from these two stages. Interestingly, eIF(iso)4E was more abundant than eIF4E in dry seeds, while both cap-binding proteins were present at similar levels in 24-h-imbibed seeds. These results suggest that the ratio of eIF(iso)4E to eIF4E in the corresponding eIF4F complex is critical for the mechanisms of translational control during maize germination.