Abstract
A hallmark of CRISPR-Cas immunity systems is the CRISPR array, a genomic locus consisting of short, repeated sequences ('repeats') interspersed with short, variable sequences ('spacers'). CRISPR arrays are transcribed and processed into individual CRISPR RNAs that each include a single spacer, and direct Cas proteins to complementary sequences in invading nucleic acid. Most bacterial CRISPR array transcripts are unusually long for untranslated RNA, suggesting the existence of mechanisms to prevent premature transcription termination by Rho, a conserved bacterial transcription termination factor that rapidly terminates untranslated RNA. We show that Rho can prematurely terminate transcription of bacterial CRISPR arrays, and we identify a widespread antitermination mechanism that antagonizes Rho to facilitate complete transcription of CRISPR arrays. Thus, our data highlight the importance of transcription termination and antitermination in the evolution of bacterial CRISPR-Cas systems.
Highlights
CRISPR-Cas systems are adaptive immune systems found in many bacteria and archaea (Wright et al, 2016)
The ability to become immune to newly encountered invaders is presumably a strong selective pressure that promotes adaptation (Bradde et al, 2020; Martynov et al, 2017), shorter CRISPR arrays appear to be strongly favored in bacteria (Weissman et al, 2018)
It has been hypothesized that increased array length is selected against because of the potential for individual CRISPR RNAs (crRNAs) to become less effective as the effector complex is diluted among more crRNA variants (Bradde et al, 2020; Martynov et al, 2017; Rao et al, 2017)
Summary
CRISPR-Cas systems are adaptive immune systems found in many bacteria and archaea (Wright et al, 2016). CrRNAs associate with an effector Cas protein or Cas protein complex, and direct the Cas protein(s) to an invading nucleic acid sequence that is complementary to the crRNA spacer and often includes a neighboring Protospacer Adjacent Motif (PAM). This leads to cleavage of the invading nucleic acid by a Cas protein nuclease, in a process known as “interference”. The ability to become immune to newly encountered invaders is presumably a strong selective pressure that promotes adaptation (Bradde et al, 2020; Martynov et al, 2017), shorter CRISPR arrays appear to be strongly favored in bacteria (Weissman et al, 2018). The copyright holder for this preprint It is made available under to be impacted by deletions caused by homologous recombination between repeats (Gudbergsdottir et al, 2011; Kupczok et al, 2015)
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