Plant ribosomes contain four specialized ribonucleic acids, the 5S, 5.8S, 18S, and 25S rRNAs. Maturation of the latter three rRNAs requires cooperative processing of a single transcript by several endonucleases and exonucleases at specific sites. In maize ( Zea mays ), the exact nucleases and components required for rRNA processing remain poorly understood. Here, we characterized a conserved RNA 3′-terminal phosphate cyclase (RCL)-like protein, RCL1, that functions in 18S rRNA maturation. R...