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 AtGRP7 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 AtGRP7 affects pri-miRNA processing. This is supported by the observation that AtGRP7 co-localizes with components of the pri-miRNA processing machinery.
RNA immunoprecipitation shows that AtGRP7, but not AtGRP7- R49Q, binds to precursors of miRNAs with reduced levels in AtGRP7-ox plants. In contrast, precursors of miRNAs that are elevated or not changed in response to high AtGRP7 levels are not bound in vivo. Thus, the in vivo binding is functionally relevant.
Besides its regulatory function pri-miRNA processing, AtGRP7 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.
Taken together, AtGRP7 is the first hnRNP-like protein in plants with a demonstrated dual role in alternative splicing of pre-mRNAs and maturation of pri-miRNAs.