RNA Biology and Molecular Physiology
A Novel Regulator of microRNA Biogenesis
MiRNAs are 21-24 nucleotide long RNAs that are processed from longer primary transcripts (pri-miRNAs) with an imperfect fold-back structure. Mature miRNAs bind to complementary mRNA target sites leading to their downregulation by cleavage or inhibition of translation.
Small RNA-seq of plants constitutively overexpressing the hnRNP-like protein At GRP7 uncovered a suite of miRNAs with reduced steady-state abundance compared to wild type plants.
Overaccumulation of pri-miRNAs at the expense of the mature miRNAs indicates that At GRP7 affects pri-miRNA processing. This is supported by the observation that At GRP7 co-localizes with components of the pri-miRNA processing machinery.
RNA immunoprecipitation shows that At GRP7, but not At GRP7- R49Q, binds to precursors of miRNAs with reduced levels in At GRP7-ox plants. In contrast, precursors of miRNAs that are elevated or not changed in response to high At GRP7 levels are not bound in vivo . Thus, the in vivo binding is functionally relevant.
Besides its regulatory function pri-miRNA processing, At GRP7 also affects the alternative splicing of pri-miRNA172b. The protein promotes splicing of the intron downstream of the stem-loop structure while inhibiting the processing.
