Peptidase-containing ABC transporters (PCATs) are a widely distributed family of transporters which secrete double-glycine (GG) peptides. In the opportunistic pathogen Streptococcus pneumoniae (pneumococcus), the PCATs ComAB and BlpAB have been shown to secrete quorum-sensing pheromones and bacteriocins related to the competence and pneumocin pathways. Here, we describe another pneumococcal PCAT, RtgAB, encoded by the rtg locus and found intact in 17% of strains. The Rgg/SHP-like quorum-sensing system RtgR/S, which uses a peptide pheromone with a distinctive Trp-X-Trp motif, regulates expression of the rtg locus and provides a competitive fitness advantage in a mouse model of nasopharyngeal colonization. RtgAB secretes a set of coregulated rtg GG peptides. ComAB and BlpAB, which share a substrate pool, do not secrete the rtg GG peptides. Similarly, RtgAB does not efficiently secrete ComAB/BlpAB substrates. We examined the molecular determinants of substrate selectivity between ComAB, BlpAB, and RtgAB and found that the GG peptide signal sequences contain all the information necessary to direct secretion through specific transporters. Secretion through ComAB and BlpAB depends largely on the identity of four conserved hydrophobic signal sequence residues previously implicated in substrate recognition by PCATs. In contrast, a motif situated at the N-terminal end of the signal sequence, found only in rtg GG peptides, directs secretion through RtgAB. These findings illustrate the complexity in predicting substrate-PCAT pairings by demonstrating specificity that is not dictated solely by signal sequence residues previously implicated in substrate recognition.IMPORTANCE The export of peptides from the cell is a fundamental process carried out by all bacteria. One method of bacterial peptide export relies on a family of transporters called peptidase-containing ABC transporters (PCATs). PCATs export so-called GG peptides which carry out diverse functions, including cell-to-cell communication and interbacterial competition. In this work, we describe a PCAT-encoding genetic locus, rtg, in the pathogen Streptococcus pneumoniae (pneumococcus). The rtg locus is linked to increased competitive fitness advantage in a mouse model of nasopharyngeal colonization. We also describe how the rtg PCAT preferentially secretes a set of coregulated GG peptides but not GG peptides secreted by other pneumococcal PCATs. These findings illuminate a relatively understudied part of PCAT biology: how these transporters discriminate between different subsets of GG peptides. Ultimately, expanding our knowledge of PCATs will advance our understanding of the many microbial processes dependent on these transporters.

中文翻译：

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肺炎球菌 Rgg 调节的含肽酶 ABC 转运蛋白底物选择性的分子决定因素。


含肽酶的 ABC 转运蛋白 (PCAT) 是分布广泛的转运蛋白家族，可分泌双甘氨酸 (GG) 肽。在机会性病原体肺炎链球菌（肺炎球菌）中，PCAT ComAB 和 BlpAB 已被证明能够分泌与能力和肺炎球菌素途径相关的群体感应信息素和细菌素。在这里，我们描述了另一种肺炎球菌 PCAT，即 RtgAB，它由 rtg 位点编码，并在 17% 的菌株中发现完整。 Rgg/SHP 类群体感应系统 RtgR/S 使用具有独特 Trp-X-Trp 基序的肽信息素，调节 rtg 基因座的表达，并在鼻咽定植小鼠模型中提供竞争性适应性优势。 RtgAB 分泌一组共同调控的 rtg GG 肽。 ComAB 和 BlpAB 共享底物池，不分泌 rtg GG 肽。类似地，RtgAB 不能有效地分泌 ComAB/BlpAB 底物。我们检查了 ComAB、BlpAB 和 RtgAB 之间底物选择性的分子决定因素，发现 GG 肽信号序列包含通过特定转运蛋白指导分泌所需的所有信息。通过 ComAB 和 BlpAB 的分泌很大程度上取决于先前参与 PCAT 底物识别的四个保守疏水信号序列残基的身份。相比之下，仅在 rtg GG 肽中发现的位于信号序列 N 末端的基序通过 RtgAB 指导分泌。这些发现通过证明特异性不仅仅由先前涉及底物识别的信号序列残基决定，说明了预测底物-PCAT配对的复杂性。重要性从细胞中输出肽是所有细菌执行的基本过程。 细菌肽输出的一种方法依赖于称为含肽酶 ABC 转运蛋白 (PCAT) 的转运蛋白家族。 PCAT 输出所谓的 GG 肽，其执行多种功能，包括细胞间通讯和细菌间竞争。在这项工作中，我们描述了病原体肺炎链球菌（肺炎球菌）中的 PCAT 编码遗传位点 rtg。 rtg 基因座与鼻咽定植小鼠模型中竞争适应性优势的增加有关。我们还描述了 rtg PCAT 如何优先分泌一组共同调节的 GG 肽，而不是其他肺炎球菌 PCAT 分泌的 GG 肽。这些发现阐明了 PCAT 生物学中相对未被充分研究的部分：这些转运蛋白如何区分 GG 肽的不同子集。最终，扩大我们对 PCAT 的了解将加深我们对依赖这些转运蛋白的许多微生物过程的理解。