Plastic pollution is ubiquitous in terrestrial and aquatic ecosystems. Plastic waste exposed to the environment creates problems and is of significant concern for all life forms. Plastic production and accumulation in the natural environment are occurring at an unprecedented rate due to indiscriminate use, inadequate recycling, and deposits in landfills. In 2019, the global production of plastic was at 370 million tons, with only 9% of it being recycled, 12% being incinerated, and the remaining left in the environment or landfills. The leakage of plastic wastes into terrestrial and aquatic ecosystems is occurring at an unprecedented rate. The management of plastic waste is a challenging problem for researchers, policymakers, citizens, and other stakeholders. Therefore, here, we summarize the current understanding and concerns of plastics pollution (microplastics or nanoplastics) on natural ecosystems. The overall goal of this review is to provide background assessment on the adverse effects of plastic pollution on natural ecosystems; interlink the management of plastic pollution with sustainable development goals; address the policy initiatives under transdisciplinary approaches through life cycle assessment, circular economy, and sustainability; identify the knowledge gaps; and provide current policy recommendations. Plastic waste management through community involvement and socio-economic inputs in different countries are presented and discussed. Plastic ban policies and public awareness are likely the major mitigation interventions. The need for life cycle assessment and circularity to assess the potential environmental impacts and resources used throughout a plastic product’s life span is emphasized. Innovations are needed to reduce, reuse, recycle, and recover plastics and find eco-friendly replacements for plastics. Empowering and educating communities and citizens to act collectively to minimize plastic pollution and use alternative options for plastics must be promoted and enforced. Plastic pollution is a global concern that must be addressed collectively with the utmost priority.

Mycobacterium tuberculosis possesses an unusual cell wall that is replete with virulence-enhancing lipids. One cell wall molecule unique to pathogenic M. tuberculosis is polyacyltrehalose (PAT), a pentaacylated, trehalose-based glycolipid. Little is known about the biosynthesis of PAT, although its biosynthetic gene cluster has been identified and found to resemble that of the better studied M. tuberculosis cell wall component sulfolipid-1. In this study, we sought to elucidate the function of papA3, a gene from the PAT locus encoding a putative acyltransferase. To determine whether PapA3 participates in PAT assembly, we expressed the protein heterologously and evaluated its acyltransferase activity in vitro. The purified enzyme catalyzed the sequential esterification of trehalose with two palmitoyl groups, generating a diacylated product similar to the 2,3-diacyltrehalose glycolipids of M. tuberculosis. Notably, PapA3 was selective for trehalose; no activity was observed with other structurally related disaccharides. Disruption of the papA3 gene from M. tuberculosis resulted in the loss of PAT from bacterial lipid extracts. Complementation of the mutant strain restored PAT production, demonstrating that PapA3 is essential for the biosynthesis of this glycolipid in vivo. Furthermore, we determined that the PAT biosynthetic machinery has no cross-talk with that for sulfolipid-1 despite their related structures.Mycobacterium tuberculosis, the bacterium that causes tuberculosis in humans, has a complex cell wall that contains a number of unique glycolipids intimately linked to mycobacterial pathogenesis (1, 2). The biosynthesis of many of these virulence factors, including the trehalose mycolates, phenolic glycolipids, and sulfolipid-1 (SL-1), 3 is largely understood (3-5). In contrast, relatively little is known about the biosynthesis of other prominent M. tuberculosis glycolipids, such as di-, tri-, and polyacyltrehaloses. These acyltrehaloses are located in the outer surface of the cell wall and contain di-and tri-methyl branched fatty acids that are only found in pathogenic species of mycobacteria (6, 7). Previous studies suggest a role for these glycolipids in anchoring the bacterial capsule, which impedes phagocytosis by host cells (6). The major polyacyltrehalose (PAT) of M. tuberculosis, also referred to as pentaacyl or polyphthienoyl trehalose, consists of five acyl chains, four mycolipenic (phthienoic) acids and one fully saturated fatty acid, linked to trehalose (Fig. 1A) (8). The mycolipenic acid side chains of PAT are products of the polyketide synthase gene pks3/4 (7). Disruption of pks3/4 (also referred to as msl3 (7)) abolishes PAT biosynthesis and causes cell aggregation. At present, the remaining proteins required for PAT assembly have not been characterized.Interestingly, the PAT biosynthetic gene cluster strongly resembles that of SL-1, which is a structurally similar trehalosebased glycolipid unique to pathogenic mycobacteria (Fig. 1B) (9). Both gene clusters contain polyketide synthase (pks), ...

Mycobacterium tuberculosis encodes for 11 eukaryotic-like serine/threonine protein kinases. Genetic and biochemical studies show that these kinases regulate various cellular processes including cell shape and morphology, glucose and glutamine transport, phagosome-lysosome fusion and the expression, and/or activity of transcription factors. PknK is the largest predicted serine/threonine protein kinase in M. tuberculosis. Here, we have cloned, overexpressed, and purified protein kinase K (PknK) to near homogeneity and show that its ability to phosphorylate proteins is dependent on the invariant lysine (Lys 55 ), and on two conserved threonine residues present in its activation loop. Despite being devoid of any apparent transmembrane domain, PknK is localized to the cell wall fraction, suggesting probable anchoring of the kinase to the cell membrane region. The pknK gene is located in the vicinity of the virS gene, which is known to regulate the expression of the mycobacterial monooxygenase (mymA) operon. We report here for the first time that VirS is in fact a substrate of PknK. In addition, four of the proteins encoded by mymA operon are also found to be substrates of PknK. Results show that VirS is a bona fide substrate of PknK in vivo, and PknK-mediated phosphorylation of VirS increases its affinity for mym promoter DNA. Reporter assays reveal that PknK modulates VirS-mediated stimulation of transcription from the mym promoter. These findings suggest that the expression of mymA operon genes is regulated through PknK-mediated phosphorylation of VirS.

Reversible protein phosphorylation is a prevalent signaling mechanism which modulates cellular metabolism in response to changing environmental conditions. In this study, we focus on previously uncharacterized Mycobacterium tuberculosis Ser/Thr protein kinase (STPK) PknJ, a putative transmembrane protein. PknJ is shown to possess autophosphorylation activity and is also found to be capable of carrying out phosphorylation on the artificial substrate myelin basic protein (MyBP). Previous studies have shown that the autophosphorylation activity of M. tuberculosis STPKs is dependent on the conserved residues in the activation loop. However, our results show that apart from the conventional conserved residues, additional residues in the activation loop may also play a crucial role in kinase activation. Further characterization of PknJ reveals that the kinase utilizes unusual ions (Ni2+, Co2+) as cofactors, thus hinting at a novel mechanism for PknJ activation. Additionally, as shown for other STPKs, we observe that PknJ possesses the capability to dimerize. In order to elucidate the signal transduction cascade emanating from PknJ, the M. tuberculosis membrane-associated protein fraction is treated with the active kinase and glycolytic enzyme Pyruvate kinase A (mtPykA) is identified as one of the potential substrates of PknJ. The phospholabel is found to be localized on serine and threonine residue(s), with Ser37 identified as one of the sites of phosphorylation. Since Pyk is known to catalyze the last step of glycolysis, our study shows that the fundamental pathways such as glycolysis can also be governed by STPK-mediated signaling.