The HY2 gene as an efficient marker for transposon excision in Arabidopsis

The two Arabidopsis RPS6 genes, encoding for cytoplasmic ribosomal proteins S6, are functionally equivalent

Many eukaryotic genomes have experienced ancient whole-genome duplication (WGD) followed by massive gene loss. These eliminations were not random since some gene families were preferentially retained as duplicates. The gene balance hypothesis suggests that those genes with dosage reduction can imbalance their interacting partners or complex, resulting in decreased fitness. In Arabidopsis, the cytoplasmic ribosomal proteins (RP) are encoded by gene families with at least two members. We have focused our study on the two RPS6 genes in an attempt to understand why they have been retained as duplicates. We demonstrate that RPS6 function is vital for the plant. We also show that reducing the level of RPS6 accumulation (in the knock-out rps6a or rps6b single mutants, or in the double heterozygous RPS6A/rps6a,RPS6B/rps6b), confers a slow growth phenotype (haplodeficiency). Importantly, we demonstrate that the functions of two RPS6 genes are redundant and interchangeable. Finally, like in most other described Arabidopsis rp mutants, we observed that a reduced RPS6 level slightly alters the dorsoventral leaf patterning. Our results support the idea that the Arabidopsis RPS6 gene duplicates were evolutionarily retained in order to maintain an expression level necessary to sustain the translational demand of the cell, in agreement with the gene balance hypothesis.

Targeted Ds-tagging strategy generates high allelic diversity at the Arabidopsis HY2 locus

The targeted (or directed) tagging is a strategy aimed to mobilize a tranposon into a specific gene (target). Only a very few Arabidopsis genes have been tagged by this way, thus the efficiency of the strategy, as well as the diversity of the alleles obtained are not well documented. We have used a maize Ds element in a directed tagging of HY2. The starting Ds element, located 22 kb proximal to HY2, has been remobilized in a cross with an Ac transposase source line. From the F2 progeny of 4800 F1 we phenotypically isolated seven hy2 mutants. Molecular analysis of these alleles revealed that two contained a Ds element in HY2 and were instable, three have a large deletion that partially or completely removed HY2, one has a footprint in a HY2 exon and one leaky allele consisted of a 22 kb inversion upstream the HY2 coding sequence. Thus, the transposon-based directed tagging strategy generates a wide diversity of tagged and non-tagged alleles that can be used to generate allelic series or deletion of clustered genes.