Department of Microbiology, University of Hawai‛i at Mānoa, Honolulu, Hawai‛i, United States of America.
PLoS One. 2018 Apr 23;13(4):e0196300. doi: 10.1371/journal.pone.0196300. eCollection 2018.
Zinc is an essential micronutrient required for proper structure and function of many proteins. Bacteria regularly encounter zinc depletion and have evolved diverse mechanisms to continue growth when zinc is limited, including the expression of zinc-independent paralogs of zinc-binding proteins. Mycobacteria have a conserved operon encoding four zinc-independent alternative ribosomal proteins (AltRPs) that are expressed when zinc is depleted. It is unknown if mycobacterial AltRPs replace their primary paralogs in the ribosome and maintain protein synthesis under zinc-limited conditions, and if such replacements contribute to their physiology. This study shows that AltRPs from Mycobacterium smegmatis are essential for growth when zinc ion is scarce. Specifically, the deletion mutant of this operon (ΔaltRP) is unable to grow in media containing a high-affinity zinc chelator, while growth of the wild type strain is unaffected under the same conditions. However, when zinc is gradually depleted during growth in zinc-limited medium, the ΔaltRP mutant maintains the same growth rate as seen for the wild type strain. In contrast to M. smegmatis grown with sufficient zinc supplementation that forms shorter cells when transitioning from logarithmic to stationary phase, M. smegmatis deficient for zinc elongates after the expression of AltRPs in late logarithmic phase. These zinc-depleted bacteria also exhibit a remarkable morphology characterized by a condensed chromosome, increased number of polyphosphate granules, and distinct appearance of lipid bodies and the cell wall compared to the zinc-replete cells. However, the ΔaltRP cells fail to elongate and transition into the zinc-limited morphotype, resembling the wild type zinc-replete bacteria instead. Therefore, the altRP operon in M. smegmatis has a vital role in continuation of growth when zinc is scarce and in triggering specific morphogenesis during the adaptation to zinc limitation, suggesting that AltRPs can functionally replace their zinc-dependent paralogs, but also contribute to mycobacterial physiology in a unique way.
锌是一种必需的微量元素，对于许多蛋白质的正确结构和功能至关重要。细菌经常面临锌耗竭的情况，并进化出多种机制，以在锌有限的情况下继续生长，包括表达锌结合蛋白的锌非依赖性同源物。分枝杆菌具有保守的操纵子，编码四个锌非依赖性替代核糖体蛋白（AltRPs），当锌耗尽时会表达这些蛋白。目前尚不清楚分枝杆菌 AltRPs 是否会在核糖体中替代其主要同源物，并在锌限制条件下维持蛋白质合成，如果这些替代物对其生理学有贡献。本研究表明，当锌离子稀缺时，来自耻垢分枝杆菌的 AltRPs 对于生长是必不可少的。具体来说，该操纵子的缺失突变体（ΔaltRP）无法在含有高亲和力锌螯合剂的培养基中生长，而在相同条件下野生型菌株的生长不受影响。然而，当在锌限制培养基中生长时逐渐耗尽锌时，ΔaltRP 突变体保持与野生型菌株相同的生长速率。与在锌充足的情况下生长的耻垢分枝杆菌形成较短细胞不同，当从对数期过渡到静止期时，锌缺乏的 M. smegmatis 伸长。与锌充足的细胞相比，这些耗尽锌的细菌还表现出显著的形态，其特征是染色体浓缩、多磷酸盐颗粒数量增加以及脂滴和细胞壁的明显外观。然而，ΔaltRP 细胞无法伸长并转变为锌限制形态，而是类似于锌充足的野生型细菌。因此，M. smegmatis 中的 altRP 操纵子在锌稀缺时继续生长以及在适应锌限制时触发特定形态发生中起着至关重要的作用，这表明 AltRPs 可以替代其锌依赖性同源物发挥功能，但也以独特的方式为分枝杆菌生理学做出贡献。
