A ruler protein in a complex for antiviral defense determines the length of small interfering CRISPR RNAs.
Small RNAs undergo maturation events that precisely determine the length and structure required for their function. CRISPRs (clustered regularly interspaced short palindromic repeats) encode small RNAs (crRNAs) that together with CRISPR-associated (cas) genes constitute a sequence-specific prokaryotic immune system for anti-viral and anti-plasmid defense. crRNAs are subject to multiple processing events during their biogenesis, and little is known about the mechanism of the final maturation step. We show that in the Staphylococcus epidermidis type III CRISPR-Cas system, mature crRNAs are measured in a Cas10·Csm ribonucleoprotein complex to yield discrete lengths that differ by 6-nucleotide increments. We looked for mutants that impact this crRNA size pattern and found that an alanine substitution of a conserved aspartate residue of Csm3 eliminates the 6-nucleotide increments in the length of crRNAs. In vitro, recombinant Csm3 binds RNA molecules at multiple sites, producing gel-shift patterns that suggest that each protein binds 6 nucleotides of substrate. In vivo, changes in the levels of Csm3 modulate the crRNA size distribution without disrupting the 6-nucleotide periodicity. Our data support a model in which multiple Csm3 molecules within the Cas10·Csm complex bind the crRNA with a 6-nucleotide periodicity to function as a ruler that measures the extent of crRNA maturation.
From the Laboratory of Bacteriology, The Rockefeller University, New York, New York 10065.
Asma
A
Hatoum-Aslan
Poulami
P
Samai
Inbal
I
Maniv
Wenyan
W
Jiang
Luciano A
LA
Marraffini
D000409:Alanine / Q000235:genetics; D000595:Amino Acid Sequence; D000900:Anti-Bacterial Agents / Q000378:metabolism; D001426:Bacterial Proteins / Q000378:metabolism*; D064113:CRISPR-Cas Systems; D064112:Clustered Regularly Interspaced Short Palindromic Repeats; D003227:Conjugation, Genetic; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000378:metabolism; D008969:Molecular Sequence Data; D009711:Nucleotides / Q000235:genetics; D010957:Plasmids / Q000378:metabolism; D012329:RNA, Bacterial / Q000235:genetics*; D034741:RNA, Small Interfering / Q000235:genetics*; D012261:Ribonucleoproteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013212:Staphylococcus epidermidis / Q000378:metabolism*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D009711:Nucleotides; D012329:RNA, Bacterial; D034741:RNA, Small Interfering; D012261:Ribonucleoproteins; D000409:Alanine
Adaptive Immunity; Antibiotic Resistance; Bacterial Conjugation; Bacterial Genetics; CRISPR; Microbiology; Small RNA; Staphylococcus