Characterization of CRISPR RNA biogenesis pathways in Streptococcus thermophilus
Carte, Jason Michael
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The CRISPR-Cas (CC) immune systems protect many prokaryotes from invasive genetic elements. These immune systems are composed of small CRISPR (cr)RNAs which guide effector Cas (CRISPR-associated) proteins to silence foreign DNA or RNA. crRNAs are derived from CRISPR arrays which are characterized by a series of short direct repeats separated by similarly sized invader-derived sequence elements known as spacers. Processing of crRNA precursors to the mature forms is a critical step in CC-mediated immunity. We have investigated the crRNA biogenesis pathways in the lactic acid bacterium Streptococcus thermophilus. S. thermophilus contains four CC systems. Each CC module is composed of a distinct CRISPR array and an adjacently encoded cluster of cas genes. We found that each CRISPR array gives rise to distinct crRNAs and that biogenesis of crRNAs from each CC system in S. thermophilus is mediated by Cas protein factors associated with that locus, independently of the other three CC modules.