Translation initiation in eukaryotes requires a number of initiation factors to recruit the assembled ribosome to mRNA. The eIF4F complex plays a key role in initiation and is a common target point for regulation of protein synthesis. Most work on the translation machinery of plants to date has focused on flowering plants, which have both the eIF4F complex (eIF4E and eIF4G) as well as the plant-specific eIFiso4F complex (eIFiso4E and eIFiso4G). The increasing availability of plant genome sequence data has made it possible to trace the evolutionary history of these two complexes in plants, leading to several interesting discoveries. eIFiso4G is conserved throughout plants, while eIFiso4E only appears with the evolution of flowering plants. The eIF4G N-terminus, which has been difficult to annotate, appears to be well conserved throughout the plant lineage and contains two motifs of unknown function. Comparison of eIFiso4G and eIF4G sequence data suggests conserved features unique to eIFiso4G and eIF4G proteins. These findings have answered some questions about the evolutionary history of the two eIF4F complexes of plants, while raising new ones. 1. Introduction In eukaryotes, posttranscriptional gene regulation at the level of translation initiation is an important mechanism [1]. The process of translation initiation begins with the eIF4F complex, made up of the subunits eIF4E, which recognizes the 7-methylguanosine (m7G) cap on the 5′ end of mRNA, and eIF4G, which binds to eIF4E and serves as a scaffold for other initiation factors [2]. eIF4G has sites for binding poly(A)-binding proteins (PABPs), which bind to the poly(A)-tail at the 3′ end of the mRNA, effectively allowing the eIF4F complex to circularize the mRNA molecule [3]. eIF4G also has RNA binding activity which may promote association with mRNA and improve eIF4E cap recognition [4]. eIF4G additionally binds the RNA helicase eIF4A [5], which promotes ATP-dependent unwinding of RNA secondary structure in a manner promoted by eIF4G and eIF4B [6]. The 43S preinitiation complex, made up of the 40S ribosomal subunit, eIF2 bound to GTP and Met-t , eIF3, eIF1, eIF1a, and eIF5 [2], is recruited to the mRNA by eIF4G through contacts with eIF3 [7] as well as eIF5 and eIF1 [8]. The docking of the 43S preinitiation complex is followed by scanning for the AUG start codon and joining of the 60S ribosomal subunit to begin translation [2]. The placement of the eIF4F complex at the beginning of this process makes it a key point for regulation of protein synthesis [9]. Flowering plants have two distinct isoforms
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