Data from: Costs of CRISPR-Cas mediated resistance in Streptococcus thermophilus

CRISPR-Cas is a form of adaptive sequence-specific immunity in microbes.
This system offers unique opportunities for the study of coevolution
between bacteria and their viral pathogens, bacteriophages. A full
understanding of the coevolutionary dynamics of CRISPR-Cas requires
knowing the magnitude of the cost of resisting infection. Here, using the
gram-positive bacterium Streptococcus thermophilus and its associated
virulent phage 2972, a well-established model system harbouring at least
two type II functional CRISPR-Cas systems, we obtained different fitness
measures based on growth assays in isolation or in pairwise competition.
We measured the fitness cost associated with different components of this
adaptive immune system: the cost of Cas protein expression, the
constitutive cost of increasing immune memory through additional spacers,
and the conditional costs of immunity during phage exposure. We found that
Cas protein expression is particularly costly, as Cas-deficient mutants
achieved higher competitive abilities than the wild-type strain with
functional Cas proteins. Increasing immune memory by acquiring up to four
phage-derived spacers was not associated with fitness costs. In addition,
the activation of the CRISPR-Cas system during phage exposure induces
significant but small fitness costs. Together these results suggest that
the costs of the CRISPR-Cas system arise mainly due to the maintenance of
the defence system. We discuss the implications of these results for the
evolution of CRISPR-Cas-mediated immunity.

CRISPR-Cas系统是微生物中一类适应性序列特异性免疫机制。该系统为探究细菌与其病毒性病原体——噬菌体之间的协同进化提供了独特契机。全面解析CRISPR-Cas系统的协同进化动力学，需明确抗感染免疫的成本量级。本研究以一套已被广泛验证的经典模型体系——携带至少两套II型功能性CRISPR-Cas系统的革兰氏阳性菌嗜热链球菌（Streptococcus thermophilus）及其烈性噬菌体2972为研究对象，通过单独培养或两两竞争的生长实验，获取了多组不同的适合度测量指标。我们针对该适应性免疫系统的不同组分测定了对应的适合度成本：包括Cas蛋白表达成本、通过额外间隔序列（spacer）增强免疫记忆的组成性成本，以及噬菌体暴露时免疫激活的条件性成本。研究发现，Cas蛋白表达的成本尤为显著——Cas缺陷型突变株的竞争能力高于携带功能性Cas蛋白的野生型菌株。通过获取至多4个噬菌体来源的间隔序列以增强免疫记忆，并未伴随显著的适合度成本。此外，噬菌体暴露时CRISPR-Cas系统的激活仅会带来小幅且显著的适合度成本。综合上述结果可知，CRISPR-Cas系统的成本主要源于防御系统的维持。本研究最后讨论了上述结果对CRISPR-Cas介导的免疫进化的启示。