RNase M is mainly involved in
RNase M5 and related maturation nucleases exemplify secondary trimming after primary endonucleolytic cleavage. While RNase III first excises individual pre-rRNAs from polycistronic transcripts, their ends remain extended by leader and trailer sequences. Secondary enzymes perform precise exonucleolytic and endonucleolytic trimming, removing surplus nucleotides, proofreading structures, and forming mature 5' and 3' ends. This staged processing couples with rRNA modification and ribosomal protein binding, ensures correct folding of functional centers, prevents degradation of imprecise transcripts
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 8, Secondary trimming nucleases in rRNA maturation