Lactococcus lactis type III-A CRISPR-Cas system cleaves bacteriophage RNA

Area of Science:

• Microbiology
• Molecular Biology
• Bacteriophage Research

Background:

• CRISPR-Cas systems provide microbial immunity against foreign nucleic acids.
• Type III-A CRISPR-Cas systems are known to target both RNA and DNA in a transcription-dependent manner.
• A specific type III-A CRISPR-Cas system was previously identified in Lactococcus lactis, conferring resistance to Siphoviridae phages.

Purpose of the Study:

• To investigate the precise mechanism of phage resistance mediated by the type III-A CRISPR-Cas system in Lactococcus lactis.
• To determine whether the system targets viral DNA or RNA during phage infection.

Main Methods:

• Constructing genetic elements in Lactococcus lactis to express CRISPR RNAs (crRNAs) complementary to phage mRNA.
• Performing in vivo nucleic acid cleavage assays to detect target degradation.
• Utilizing Northern blot analysis to monitor phage mRNA cleavage in infected cells.

Main Results:

• Phage resistance was conferred only when crRNAs were complementary to phage mRNA, not phage DNA.
• In vivo assays did not detect cleavage of the phage double-stranded DNA (dsDNA) genome.
• Northern blots confirmed that the lactococcal CRISPR-Cas system cleaves phage mRNA within infected cells, impairing phage DNA replication.

Conclusions:

• The type III-A CRISPR-Cas system in Lactococcus lactis functions by targeting and cleaving phage mRNA.
• This mRNA-targeting mechanism represents a key strategy for bacterial defense against viral infection.
• While direct dsDNA targeting was not observed, the system effectively inhibits phage propagation through mRNA degradation.