A transcript encoding translation initiation factor eIF-5A is stored in unfertilized egg cells of maize

Differential screening of cDNA libraries of unfertilized egg cells and in v itro zygotes of maize resulted in the isolation of more than 50 different,g enes whose expression is up- or down-regulated after in vitro fertilization (IVF). Amoung these genes, we identified a cDNA encoding the eukaryotic tr anslation initiation factor eIF-5A. This highly conserved factor is thought to be necessary for selective mRNA stabilization and translation. It is al so the only known protein that contains the unusual amino acid hypusine whi ch is required for biological activity. High transcript amounts are stored in the egg cell, which is, in terms of metabolism, relatively inactive. Upo n fertilization transcript amounts decrease, in contrast to metabolically i nactive embryos in which the transcript cannot be detected and transcript l evels increase upon germination. The expression pattern during the first em bryonic cell cycle is also different from that observed during the somatic cell cycle: egg cells in the GO phase contain high transcript levels, while arrested suspension cells contain few transcripts. In the somatic cell cyc le, eif-5A is strongly induced during the G1 phase and transcripts are cont inuously degraded during the S, G2 and M phases until new induction during the G1 phase of the next cycle. eif-5A, a member of a small gene family in maize, is expressed in most maize tissues investigated. Based on our result s, we suggest that the unfertilized egg cell of maize, although relatively inactive regarding its metabolism, is prepared for selective mRNA translati on that is quickly triggered after fertilization. We also suggest that the regulation of eif-5A in the first embryonic cell cycle is different from th e somatic cell cycle.