INTRODUCTION The carriers of protein translation in eukaryotic cells, the ribosomes, have a complex structure consisting of two subunits – the 60S (Large) and 40S (Small) subunit – that are each built from a considerable number of ribosomal proteins (RPs) and ribosomal RNAs (Moore, 1998). From one of the most well- studied models for ribosome structure and constitution, the rat, we now know that eukaryotic ribosomes consist of 82 different, highly conserved proteins (Wool et al., 1995). The importance of the protein translation machinery in a given process can be studied by analysing the effect of a single rp mutation, as the absence of a single RP prevents assembly of the corresponding ribosomal subunit (Volarevic ´ et al., 2000). Notably, apart from expected defects such as growth arrest or delay, mutations in RP genes often result in specific defects in the development of an organism (Wool, 1996). In some cases, RP gene mutations affect DNA replication, RNA processing and DNA repair (Wool, 1996), suggesting roles for RPs that are additional to protein translation. One of the most prominent examples of specific developmental defects due to RP gene mutations is the Minute syndrome in Drosophila. The Minute class of mutants was described, as early as in 1919 (Lambertsson, 1998), and today over 50 different Minute loci have been mapped. The Minute syndrome is characterised by semi-dominant phenotypes, such as delay in larval development, smaller body size and several pleiotropic morphological aberrations (e.g. short thoracic bristles), and recessive embryo lethality. Kongsuwan and co- workers (Kongsuwan et al., 1985) first showed that a Minute phenotype was caused by a deletion of a RP gene, and now, at least 11 Minute loci have been assigned to RP genes (Lambertsson, 1998), demonstrating that ribosome function is essential during specific stages of fly development and that individual RPs can easily become rate-limiting for various processes. Only few mutations in RP genes have been described in the plant Arabidopsis thaliana. Two such mutations were identified during screens for either aberrant seedling phenotypes (van Lijsebettens et al., 1994) or altered sensitivity to genotoxic stress (Revenkova et al., 1999). A third rp mutant, identified through sequence analyses of Ds transposon insertion sites, also appeared to show specific defects in seedling development (Ito et al., 2000). Strikingly, all three of the above rp phenotypes were found to be recessive. Semi- dominant phenotypes, as described for the Drosophila Minute mutants, have not been reported in plants. This might be a consequence of the fact that RP genes are single copy in Drosophila (Kay and Jacobs-Lorena, 1987), while in Arabidopsis each RP is represented by a small gene family 4289 Development 128, 4289-4299 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV0353 Mutations in ribosomal protein (RP) genes in Drosophila lead to strong developmental phenotypes, expressed in the semi-dominant Minute syndrome. In plants, however, mutations in RP genes have so far only been reported to result in recessive developmental phenotypes. We present the analysis of an Arabidopsis promoter-trap line, in which a T-DNA insertion in an RPS5 gene (AtRPS5A) causes semi- dominant developmental phenotypes. Most cell-division processes are delayed or disturbed in the heterozygous mutant, and development is completely arrested at an early embryonic stage in the homozygous mutant. By analogy with Drosophila rp mutants, we have named this mutant Arabidopsis Minute-like 1 (aml1). As with other Arabidopsis RPs, RPS5 is represented by a small gene family, but in contrast to other described plant RPs, this family comprises only two members. The AtRPS5A gene (mutated in aml1) is strongly expressed in dividing cells, whereas expression of the second RPS5 gene, AtRPS5B, is lower than that of AtRPS5A, and is correlated with cell differentiation rather than cell division. From expression analyses we conclude that AtRPS5A is the most abundantly expressed RPS5 gene in Arabidopsis. The Minute-like defects in the aml1 mutant provide the first evidence that ribosome insufficiency leads to similar consequences in both plants and insects, and emphasize the general importance of efficient protein translation for cell proliferation in higher eukaryotes. Key words: Arabidopsis, Semi-dominant mutation, Cell division, Ribosomal protein, Minute syndrome, Haplo-insufficiency SUMMARY An Arabidopsis Minute-like phenotype caused by a semi-dominant mutation in a RIBOSOMAL PROTEIN S5 gene Dolf Weijers, Marry Franke-van Dijk, Robert-Jan Vencken, Ab Quint, Paul Hooykaas and Remko Offringa* Institute of Molecular Plant Sciences, Leiden University, Clusius Laboratory, Wassenaarseweg 64, 2333 AL, Leiden, The Netherlands *Author for correspondence (e-mail: offringa@rulbim.leidenuniv.nl) Accepted 8 August 2001