Abstract
Ribosome biogenesis is well described in Saccharomyces cerevisiae. In contrast only very little information is available on this pathway in plants. This study presents the characterization of five putative protein co-factors of ribosome biogenesis in Arabidopsis thaliana, namely Rrp5, Pwp2, Nob1, Enp1 and Noc4. The characterization of the proteins in respect to localization, enzymatic activity and association with pre-ribosomal complexes is shown. Additionally, analyses of T-DNA insertion mutants aimed to reveal an involvement of the plant co-factors in ribosome biogenesis. The investigated proteins localize mainly to the nucleolus or the nucleus, and atEnp1 and atNob1 co-migrate with 40S pre-ribosomal complexes. The analysis of T-DNA insertion lines revealed that all proteins are essential in Arabidopsis thaliana and mutant plants show alterations of rRNA intermediate abundance already in the heterozygous state. The most significant alteration was observed in the NOB1 T-DNA insertion line where the P-A3 fragment, a 23S-like rRNA precursor, accumulated. The transmission of the T-DNA through the male and female gametophyte was strongly inhibited indicating a high importance of ribosome co-factor genes in the haploid stages of plant development. Additionally impaired embryogenesis was observed in some mutant plant lines. All results support an involvement of the analyzed proteins in ribosome biogenesis but differences in rRNA processing, gametophyte and embryo development suggested an alternative regulation in plants.
Highlights
Ribosome biogenesis requires the coordination of roughly 200 protein co-factors that assist in 60S and 40S subunit assembly and ribosomal RNA processing [1,2,3,4]
We have searched for orthologs in Arabidopsis thaliana and named the identified factors according to the yeast standard name
Based on sequence similarity we assume that the selected plant factors are involved in ribosome biogenesis the Arabidopsis proteins could not complement yeast depletion phenotypes
Summary
Ribosome biogenesis requires the coordination of roughly 200 protein co-factors that assist in 60S and 40S subunit assembly and ribosomal RNA (rRNA) processing [1,2,3,4]. Plant homoloques to yeast proteins like the eukaryotic translation initiation factor 6 (eIf6), the exoribonuclease 2 (Xrn2) or rRNA processing co-factors nucleolar complex associated protein 1 (Noc1)/maintenance of killer 21 (Mak21) have been identified [8,9,10,11]. Domino homozygous deletion mutants arrest early in embryogenesis in the globular stage and show enlarged nucleoli in the embryo and the endosperm. Based on this and subsequent studies it is assumed that alterations in nucleolar structure and defects in embryogenesis are phenotypes associated with impaired ribosome biogenesis [13,14,15]. The ribosome biogenesis pathway appears to be an ideal subject for investigation of embryogenesis and defects thereof caused by malfunction of factors involved
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