Abstract
Clustered regularly interspaced short palindromic repeat (CRISPR) systems and prokaryotic Argonaute proteins (Agos) have been shown to defend bacterial and archaeal cells against invading nucleic acids. Indeed, they are important elements for inhibiting horizontal gene transfer between bacterial and archaeal cells. The CRISPR system employs an RNA-guide complex to target invading DNA or RNA, while Agos target DNA using single stranded DNA or RNA as guides. Thus, the CRISPR and Agos systems defend against exogenous nucleic acids by different mechanisms. It is not fully understood how antagonization of these systems occurs during natural transformation, wherein exogenous DNA enters a host cell as single stranded DNA and is then integrated into the host genome. In this review, we discuss the functions and mechanisms of the CRISPR system and Agos in cellular defense against natural transformation.
Highlights
Horizontal exchange of DNA between bacteria is an important mechanism to generate genome diversity and drive evolution (Gogarten and Townsend, 2005)
The repeat/spacer arrays are transcribed as long Clustered regularly interspaced short palindromic repeat (CRISPR) RNA precursors that are cleaved within the repeat sequences and transformed into small CRISPR RNAs (crRNAs) by Cas endoribonucleases (Hatoum-Aslan et al, 2011), which base pair with foreign DNA via spacer-encoded sequences (Brouns et al, 2008)
The third step is the interference phase, wherein crRNAs are used as antisense guides that combine with sets of Cas proteins to form the core CRISPR-Cas ribonucleoprotein complexes
Summary
Horizontal exchange of DNA between bacteria is an important mechanism to generate genome diversity and drive evolution (Gogarten and Townsend, 2005). The emergence of super resistant, virulent bacterial strains has largely been inferred to be caused by the transfer of antimicrobial resistance and virulence genes among different species (Maeusli et al, 2020). The acquisition of genetic material can occur through natural transformation, direct DNA uptake from the environment, conjugation, plasmid transfer from other cells, and transduction, the latter of which incorporates heterologous DNA from bacteriophage infection (Arber, 2014; Darmon and Leach, 2014). Only natural transformation is exclusively facilitated by genes from the bacterial chromosome (Ambur et al, 2016)
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