ZmSSRP1 facilitates the progression of RNA polymerase II transcription and is essential for kernel development in maize

RNA polymerase II (RNAP II) transcription through nucleosomes is assisted by the protein complex FACT (facilitates chromatin transcription). However, the precise mechanisms by which FACT regulates the distribution and progression of RNAP II in plants remain largely unclear. Here, we identify a crucial FACT subunit, SSRP1 (structure specific recognition protein 1), based on the characterization of the defective kernel 56 (dek56) mutant and demonstrate that it plays a crucial role in RNAP II transcription and seed development in maize (Zea mays). Using genome-wide mapping analysis, we found that ZmSSRP1 binding sites co-localize with RNAP II and are highly enriched within actively transcribed genes. Notably, dysfunction of ZmSSRP1 results in an early arrest of elongation, which is dependent on the length of the gene and coincides with a significant compression of the elongation rate. Furthermore, the amount of RNAP II is reduced in the dek56 mutant, which could be reverted by inhibiting the proteasome, indicating proteasomal degradation of arrested RNAP II. These findings bring to light the indispensable role for ZmSSRP1 in the regulation of RNAP II transcription, which is critical for the proper expression of thousands of genes during maize seed development.