The Maize Imprinted Gene <i>Floury3</i> Encodes a PLATZ Protein Required for tRNA and 5S rRNA Transcription through Interaction with RNA Polymerase III

Abstract

Maize (<i>Zea mays</i>) <i>floury3</i> (<i>fl3</i>) is a classic semidominant negative mutant that exhibits severe defects in the endosperm but <i>fl3</i> plants otherwise appear normal. We cloned the <i>fl3</i> gene and determined that it encodes a PLATZ (plant AT-rich sequence and zinc binding) protein. The mutation in <i>fl3</i> resulted in an Asn-to-His replacement in the conserved PLATZ domain, creating a dominant allele. <i>Fl3</i> is specifically expressed in starchy endosperm cells and regulated by genomic imprinting, which leads to the suppressed expression of <i>fl3</i> when transmitted through the male, perhaps as a consequence the semidominant behavior. Yeast two-hybrid screening and bimolecular luciferase complementation experiments revealed that FL3 interacts with the RNA polymerase III subunit 53 (RPC53) and transcription factor class C 1 (TFC1), two critical factors of the RNA polymerase III (RNAPIII) transcription complex. In the <i>fl3</i> endosperm, the levels of many tRNAs and 5S rRNA that are transcribed by RNAPIII are significantly reduced, suggesting that the incorrectly folded fl3 protein may impair the function of RNAPIII. The transcriptome is dramatically altered in <i>fl3</i> mutants, in which the downregulated genes are primarily enriched in pathways related to translation, ribosome, misfolded protein responses, and nutrient reservoir activity. Collectively, these changes may lead to defects in endosperm development and storage reserve filling in <i>fl3</i> seeds.