Maize ZmBCCIP facilitates cleavage at the A3 site in pre-rRNA processing and is crucial to seed development and...

The precise cleavage and processing of precursor rRNA (pre-rRNA) are essential for ribosome biosynthesis, a fundamental process for cell proliferation and growth. However, the molecular mechanisms underlying pre-rRNA processing in plants remain incompletely understood. In this study, we report the role of ZmBCCIP in pre-rRNA processing, which facilitates cleavage at the A3 site in the "ITS1-first" pathway by interacting with components of the mitochondrial RNA processing (MRP) endoribonuclease complex. ZmBCCIP is a nucleolar protein that is highly conserved across yeast, humans, and plants. A loss of ZmBCCIP function delays both maize seed development and seedling growth. Analysis of pre-rRNA processing in zmbccip mutants revealed an inhibition of cleavage at the A3 site of 35S pre-rRNA, leading to a reduction in the levels of 27S-A3, P-A3, and P'-A3 pre-rRNA intermediates, and an accumulation of 35S, 33S, and 27S-A2 intermediates. The decreased cleavage at the A3 site primarily impacts the processing of mature 25S rRNA, thereby hindering 60S ribosome assembly while exerting a lesser effect on the production of 18S rRNA and the formation of 40S ribosomes. The processing of 18S rRNA is maintained due to the compensatory effects of the "5'ETS-first" pathway, highlighting the adaptive advantage of having dual rRNA processing pathways in plants. ZmBCCIP interacts with ZmRPL23, which has a conserved role in shuttling and also associates with SHREK1, a WD40 protein involved in A3 cleavage, along with several components of the RNase MRP complex that mediates A3 cleavage in yeast. These findings highlight the role of BCCIP in facilitating A3 site cleavage during pre-rRNA processing and its crucial function in maize seed development and plant growth.