Ribonuclease III

The RNase III superfamily is divided into four known classes: 1, 2, 3, and 4. Each class is defined by its domain structure.[6]

Class 1 RNase III

• Class 1 RNase III enzymes have a homodimeric structure whose function is to cleave dsRNA into multiple subunits. It is a Mg²⁺-dependent endonuclease and is largely found in bacteria and bacteriophage. Class 1 RNase III have been found in Glomeromycotan fungi, which was suspected to be the result of horizontal gene transfer from cyanobacteria.[7] Among the RNases III in the class are the rnc from E. coli. Typically, class I enzymes possess a single RNase III domain (RIIID) followed by a dsRNA-binding domain (dsRBD).[6] They process precursors to ribosomal RNA, small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA). The basic dsRNA cleavage function of Class 1 RNase III is retained in most of the organisms in which it is present. However, in a number of species the function has changed and taken on different or additional biological roles.[8]

Rnc (UniProtKB P0A7Y0) - E.Coli - this RNase III is involved in the processing of viral transcripts and some mRNAs through the cleavage of multiple areas on the dsRNA. This cleavage can be influenced by ribosomal protein presence.[9]

The variances of Class 1 RNase III, called Mini-III, are homodimeric enzymes and consist solely of the RNase III domains.[10]

Class 2 RNase III

Class 2 ribonuclease III (Rnt1p) from Saccharomyces cerevisiae in complex with double-stranded RNA

• Class II is defined by the presence of an N-terminal domain (NTD), a RIIID, and a dsRBD. Class II is found in some species of Fungi.[6] They process precursors to ribosomal RNA, small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA)

Yeast nucleases with the Class 2 RNase III domain:[11]

RNT1 (UniProtKB Q02555) - S. cerevisiae - this RNase III is involved in the transcription and processing of rDNA, the 3' end formation of U2 snRNA via cleavage of the terminal loop, cell wall stress response and degradation, and regulation of morphogenesis checkpoint genes.[12]

Pac1 (UniProtKB P22192) - S. pombe - this RNase III is located on chromosome II of the yeast nuclease and when over expressed, is directly involved in the sterility, lack of mating efficiency, abnormal mitotic cell cycle, and mutation suppression of the organism.[13]

Class 3 RNase III

The crystal structure of the human Drosha ribonuclease enzyme in complex with two C-terminal helices of the DGCR8 protein.

• Class 3 RNases III include the Drosha family of enzymes known to function in maturation of precursors to miRNA.[14]

Class 4 RNase III

• Class 4 RNases III include the Dicer family of enzymes known to function in RNA interference (RNAi).[15] Class III RNases are S-RNase components. It is a component of the self-incompatibility system in Rosaceae, Solanaceae, and Plantaginaceae. They are recruited to cope with various environmental stress scenarios.[16]
• Dicer enzymes process dsRNA subtrates into small RNA fragments of individual size ranging from 21-27 nucleotides in length.[17] Dicer has an N-terminal helicase/ATPase domain which is followed by another domain of an unknown function. It is centrally positioned PAZ domain and a C-terminal configuration which includes one dsRBD and two RNase III catalytic domains.[18] Interactions of Dicer occurs with other proteins, which includes TRBP, PACT, and Ago2.[19] RNAs that are produced by Dicer act as guides for a sequence of particular silencing of cognate genes through RNAi and related pathways.[17]

• DICER1[20]
• DROSHA[21]

• RncO

• RNase+III at the US National Library of Medicine Medical Subject Headings (MeSH)
• EC 3.1.26.3