Département de biochimie, microbiologie, et bio-informatique, Faculté des sciences et de génie, Université Laval, Québec City, QC, Canada.
Groupe de recherche en écologie buccale, Faculté de médecine dentaire, Université Laval, Québec City, QC, Canada.
Nat Commun. 2022 May 19;13(1):2802. doi: 10.1038/s41467-022-30310-x.
CRISPR-Cas systems in prokaryotic cells provide an adaptive immunity against invading nucleic acids. For example, phage infection leads to addition of new immunity (spacer acquisition) and DNA cleavage (interference) in the bacterial model species Streptococcus thermophilus, which primarily relies on Cas9-containing CRISPR-Cas systems. Phages can counteract this defense system through mutations in the targeted protospacers or by encoding anti-CRISPR proteins (ACRs) that block Cas9 interference activity. Here, we show that S. thermophilus can block ACR-containing phages when the CRISPR immunity specifically targets the acr gene. This in turn selects for phage mutants carrying a deletion within the acr gene. Remarkably, a truncated acrIIA allele, found in a wild-type virulent streptococcal phage, does not block the interference activity of Cas9 but still prevents the acquisition of new immunities, thereby providing an example of an ACR specifically inhibiting spacer acquisition.
原核细胞中的 CRISPR-Cas 系统为抵御入侵的核酸提供了一种适应性免疫。例如，噬菌体感染会导致细菌模式物种嗜热链球菌中新的免疫（间隔获取）和 DNA 切割（干扰），主要依赖于含有 Cas9 的 CRISPR-Cas 系统。噬菌体可以通过靶向原间隔区的突变或通过编码抗 CRISPR 蛋白（ACRs）来阻止 Cas9 的干扰活性来对抗这种防御系统。在这里，我们表明，当 CRISPR 免疫特异性靶向 acr 基因时，嗜热链球菌可以阻止含有 ACR 的噬菌体。这反过来又选择了携带 acr 基因缺失的噬菌体突变体。值得注意的是，在一种野生型毒力链球菌噬菌体中发现的截断的 acrIIA 等位基因不会阻断 Cas9 的干扰活性，但仍能阻止新免疫的获取，从而为专门抑制间隔获取的 ACR 提供了一个例子。
