Many bacteria encode biosynthetic proteins that produce a vast array of natural products. These compounds are often synthesized during host invasion as they function as virulence factors. In addition, such secondary metabolites have yielded numerous molecular scaffolds with pharmaceutical and clinical importance. The gene clusters that encode proteins responsible for synthesis of these compounds are typically silenced or “cryptic” under laboratory growth conditions, hampering discovery of novel lead compounds. We report here that MftR is a global repressor of secondary metabolite synthesis in Burkholderia thailandensis and that urate functions as a physiologically relevant inducer of gene expression. Biosynthetic gene clusters under MftR control include those associated with production of the antimicrobial bactobolins, the iron siderophore malleobactin, and the virulence factor malleilactone. MftR also controls additional genes associated with survival in a host environment, such as genes encoding components of the type III secretion system (T3SS) and proteins linked to anaerobic respiration. This observation not only has implications for understanding activation of gene regulatory networks during host invasion, but it also paves the way for isolation of novel therapeutic leads.

SUMMARY Species within the genus Burkholderia exhibit remarkable phenotypic diversity. Genomic plasticity, including genome reduction and horizontal gene transfer, has been correlated with virulence traits in several species. However, the conservation of virulence genes in species otherwise considered to have limited potential for infection suggests that phenotypic diversity may not be explained solely on the basis of genetic diversity. Instead, differential organization and control of gene regulatory networks may underlie many phenotypic differences. In this review, we evaluate how regulation of gene expression by members of the multiple antibiotic resistance regulator (MarR) family of transcription factors may contribute to shaping the physiological diversity of Burkholderia species, with a focus on the clinically relevant human pathogens. All Burkholderia species encode a relatively large number of MarR proteins, a feature common to bacteria that must respond to environmental changes such as those associated with host invasion. However, evolution of gene regulatory networks has likely resulted in orthologous transcription factors controlling disparate sets of genes. Adaptation to, and survival in, diverse habitats, including a human or plant host, is key to the success of Burkholderia species as (opportunistic) pathogens, and recent reports suggest that control of virulence-associated genes by MarR proteins features prominently among the survival strategies employed by these species. We suggest that identification of MarR regulons will contribute significantly to clarification of virulence determinants and phenotypic diversity.

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Male germ cells are sensitive to heat stress and testes must be maintained outside the body for optimal fertility. However, no germ cell intrinsic mechanism that protects from heat has been reported. Here, we identify the germ cell specific Golgi glycoprotein MGAT4D as a protector of male germ cells from heat stress. Mgat4d is highly expressed in spermatocytes and spermatids. Unexpectedly, when the Mgat4d gene was inactivated globally or conditionally in spermatogonia, or mis-expressed in spermatogonia, spermatocytes or spermatids, neither spermatogenesis nor fertility were affected. On the other hand, when males were subjected to mild heat stress of the testis (43 °C for 25 min), germ cells with inactivated Mgat4d were markedly more sensitive to the effects of heat stress, and transgenic mice expressing Mgat4d were partially protected from heat stress. Germ cells lacking Mgat4d generally mounted a similar heat shock response to control germ cells, but could not maintain that response. Several pathways activated by heat stress in wild type were induced to a lesser extent in Mgat4d[−/−] heat-stressed germ cells (NFκB response, TNF and TGFβ signaling, Hif1α and Myc genes). Thus, the Golgi glycoprotein MGAT4D is a novel, intrinsic protector of male germ cells from heat stress. MGAT4D is designated family member D of the MGAT4 gene family by the Human Genome Nomenclature Committee based on sequence similarity to other members, including MGAT4A and MGAT4B. The latter are N-acetylglucosaminyltransferases (GlcNAcTs) that add a β1, 4GlcNAc to complex N-glycans. However, when MGAT4D is transfected into cultured cells, it does not appear to have GlcNAcT activity. Rather, it inhibits MGAT1 activity, the GlcNAcT responsible for initiating complex N-glycan synthesis 1. Because of this inhibitory activity, the protein was termed GnT1IP for GlcNAcT1 Inhibitory Protein. The Mgat4d gene is highly expressed in mouse testis with little expression in other mouse tissues 2. Based on RNA-seq analysis, it is expressed in spermatocytes and spermatids, but not in spermatogonia, sperm or Sertoli cells 3. MGAT4D is the most abundant protein in purified Golgi from rat testis germ cells 4. Characterization of the interactions of MGAT4D in the Golgi using a fluorescence resonance energy transfer (FRET) assay showed that it interacts with MGAT1 but not MGAT2, MGAT3, MGAT4B or MGAT5 3. Since knockout of Mgat1 in spermatogonia disrupts spermatogenesis and results in infertility 5,6 , deletion or overexpression of Mgat4d in germ cells were both expected to have effects on spermatogenesis. In this paper, we show that unexpectedly, deletion of Mgat4d globally, or specifically in spermatogonia, or mis-expression of Mgat4d in spermatogonia, spermatocytes or spermatids, do not appear to alter spermatogenesis in young or aged mice, and do not affect fertility. However, mild heat stress of the testis in aged mice revealed that germ cells lacking Mgat4d exhibited more damage and apoptosis following heat stress. By contrast, a Mgat4d transgene expressed i...