Eukaryotic translation initiation factor 6 (eIF6) is an essential component of ribosome biogenesis. In our present study, we characterize plant eIF6 genes for the first time. Although a single gene encodes eIF6 in yeast and animals, two genes were found to encode proteins homologous to animal and yeast eIF6 in Arabidopsis and rice, denoted At-eIF6:1 and At-eIF6:2, and Os-eIF6; 1 and Os-eIF6:2, respectively. Analysis of the yeast eif6 (tif6) mutant suggested that plant eIF6, at least in the case of At-eIF6:1, can complement the essential function of eIF6 in yeast. Evidence for the essential role of eIF6 in plants was also provided by the embryonic-lethal phenotype of the at-eIf6; 1 mutant. In contrast, At-eIF6:2 appears not to be essential due to its very low expression level and the normal growth phenotype of the eif6:2 mutants. Consistent with the putative role of plant eIF6 in ribosome biogenesis, At-eIF6:1 is predominately expressed in tissues where cell division actively proceeds under the control of intronic cis-regulatory elements. On the other hand, both Os-eIF6;1 and Os-eIF6;2 are probably active genes because they are expressed at significant expression levels. Interestingly, the supply of ammonium nitrate as a plant nutrient was found to induce specifically the expression of Os-eIF6;2. Our present findings indicate that the eIF6 genes have differently evolved in plant and animal kingdoms and also in distinct plant species. (C) 2010 Elsevier Inc. All rights reserved.