MicroRNAs (miRNAs) are ubiquitous controllers of gene expression in plants, animals, and humans. They function by guiding repressive protein complexes to "target" messenger RNAs (mRNAs) by base-pairing with partially complementary sequences. Several plant miRNAs, as well as their target relationships, have persisted unchanged in multiple lineages over very long time spans. However, due to both their pervasive redundancy and small size, it has been difficult to directly ascertain the functions of these miRNAs by straightforward mutational analyses. Using a recently described strategy to overcome these hurdles, the functions of the most ancient plant miRNAs will be directly inferred by loss of function analyses in two highly divergent species. Intriguingly, most known targets of ancient plant miRNAs are mRNAs encoding DNA-binding transcription factors, implying that the ancient miRNAs ultimately control downstream suites of gene expression. To begin to understand the transcriptional networks controlled by miRNA- targeted transcription factors, genome-wide DNA-binding sites for a representative factor will be determined using chromatin immunoprecipitation coupled to highly parallel, cloning-independent DNA sequencing. These experiments will determine the genome-wide occupancy patterns of orthologous, miR166-targeted Homeodomain transcription factors in the flowering plant Arabidopsis thaliana and in the moss Physcomitrella patens. The combined results of these experiments will provide compelling insight into the evolution and functions of ancient miRNAs.
Arnold O. Beckman exemplifies the meaning of the word humanitarian. Combined with his unwavering enthusiasm for life, his keen sense of humor and his strong moral and ethical principles, he is a national icon.