Signals and regulators that govern Streptomyces development

Abstract

Streptomyces coelicolor is the genetically best characterized species of a populous genus belonging to the gram-positive Actinobacteria. Streptomycetes are filamentous soil organisms, well known for the production of a plethora of biologically active secondary metabolic compounds. The Streptomyces developmental life cycle is uniquely complex and involves coordinated multicellular development with both physiological and morphological differentiation of several cell types, culminating in the production of secondary metabolites and dispersal of mature spores. This review presents a current appreciation of the signaling mechanisms used to orchestrate the decision to undergo morphological differentiation, and the regulators and regulatory networks that direct the intriguing development of multigenomic hyphae first to form specialized aerial hyphae and then to convert them into chains of dormant spores. This current view of S. coelicolor development is destined for rapid evolution as data from '-omics' studies shed light on gene regulatory networks, new genetic screens identify hitherto unknown players, and the resolution of our insights into the underlying cell biological processes steadily improve.

Figure 1

Visual appearance of the surfaces of wild type and bald mutant colonies. Wild type S. coelicolor colonies differentiate and form an aerial mycelium with a fuzzy gray appearance due to the conversion of aerial hyphae into chains of gray-pigmented spores. In contrast, the bald mutant, with a mutation in the adpA (bldH) gene, produces smooth and lustrous colonies lacking an aerial mycelium. For this particular bld mutant, the red-pigmented antibiotic undecylprodigiosin is still produced. Colonies were grown for 5 days on rich medium (R2YE) at 30°C. This figure was adapted from (Nguyen, et al., 2003) with permission from American Society for Microbiology.

Figure 2

Cell surface of spores derived from aerial hyphae and the extracellular matrix of vegetative hyphae involved in attachment to hydrophobic surfaces. Panel A, A TEM image of a portion of a chain of mature spores from a wild type strain is shown. The thickened spore walls are surrounded by vestiges of a superficial fibrous sheath (black arrow). Image reprinted from (Hopwood, et al., 1970) with permission from Society for General Microbiology. Panel B, The SEM image shows that chaplin fibers assemble into paired rodlet structures on the surface of the spore. The assembly is dependent on the rodlin proteins, but the rodlins are not structural components of the structures. Image reproduced from (Di Berardo, et al., 2008) with permission from American Society for Microbiology. Panels C and D, Formation of fimbriae in extracellular matrix of vegetative hyphae are revealed by TEM with negative staining. Fimbriae are formed in the extracellular matrix in a minimal medium with mannitol, but not glucose, as carbon source. Arrows in D highlight that fibers emanate from spiked protrusions. Scale bar in C is 5 μm for the image in panel C, 100 nm for inset panel of C and 1.25 μm for panel D. Images reproduced from (de Jong, et al., 2009a) with permission from the authors.

Figure 3

Stages in the differentiation of aerial hyphae into chains of spores. (A) Schematic representation of the conversion of the apical sporogenic cell of an aerial hypha into a chain of spores. A basal septum delimits the sporogenic cell from the underlying cell (referred to as a “subapical stem”), and provides the ability to have compartment-specific gene expression. The developmental upregulation of FtsZ production in the sporogenic cell, the assembly of FtsZ into helical filaments, and then shorter spirals coalesce into regularly positioned Z rings are indicated (green). Synchronous division and metamorphosis of the resulting string of single-celled compartments results in a chain of spores. Image reproduced from (Grantcharova, et al., 2005) with permission from American Society for Microbiology. (B) The production and nucleoid-localization of a HupS-EGFP fusion protein is used as a molecular marker of the sporogenic apical cells. The hypha labeled 1 shows an aerial hyphal branch in which a clear distinction is seen between the sporogenic apical cell in which hupS is expressed and a short subapical stem. In hypha 2, sporulation septation is in progress in the sporogenic cell, as indicated by the separation of individual nucleoids. In hypha 3, constrictions indicate that septation has been completed and spores are rounding up while laying down a thick spore wall. A released mature spore with a condensed nucleoid is also indicated (4). HupS-EGFP fluorescence is shown in green overlay on a phase-contrast image. Scale bar, 8 μm. Image reproduced and amended from (Salerno, et al., 2009) with permission from American Society for Microbiology.

Figure 4

Summary of an early part of the developmental regulatory pathways in S. coelicolor, centered on the regulator AdpA and leading to aerial mycelium formation. Demonstrated direct interactions at transcriptional, translational, or post-translational levels are shown as solid lines. Filled arrows indicate positive or activating effects, perpendicular lines indicate negative or repressing effects, and lines ending in bullet indicate that direct interaction has been demonstrated, but the effect on gene expression is not clear. In the case of STI, it is known that expression of sti is strongly decreased in a bldD mutant, but it has not been clarified whether this is mediated by the direct binding of BldD to the promoter region of sti (open arrowhead), or via an indirect effect. One BldD target that is not mentioned in the text is bldC, encoding a MerR-family transcription factor that is conditionally required for aerial mycelium formation (Hunt, et al., 2005). Dashed lines indicate hypothetical interactions. The effects of AdpA on STI and σ^BldN are based on genetic data and extrapolations from data for S. griseus. Dotted arrows not pointing to symbols are used to indicate unknown but anticipated target genes. The question mark indicates unknown mechanisms for counteracting BldD activity.

Figure 5

Summary of the regulatory connectivity in the intersection of aerial mycelium formation and early events in the differentiation of aerial hyphae in S. coelicolor. Demonstrated direct interactions at transcriptional, translational, or post-translational levels are shown as solid lines. Filled arrows indicate positive or activating effects, perpendicular lines indicate negative or repressing effects, and lines ending in bullet indicate that direct interaction has been demonstrated, but the effect on gene expression is not clear. Dashed lines indicate hypothetical interactions. The question marks indicate unknown mechanisms for counteracting BldD and BldG activity, and the still hypothetical sigma factor controlled by anti-sigma factor ApgA. Dotted arrows not pointing to symbols are used to indicate unknown but anticipated target genes.

Figure 6

Schematic summary of key regulators required for further differentiation of aerial hyphae into spores in S. coelicolor. Demonstrated direct interactions at transcriptional, translational, or post-translational levels are shown as solid lines. Filled arrows indicate positive or activating effects, perpendicular lines indicate negative or repressing effects, and lines ending in bullet indicate that direct interaction has been demonstrated, but the effect on gene expression is not clear. Dashed lines indicate hypothetical interactions. The question marks indicate unknown mechanisms for counteracting BldD activity. Dotted arrows not pointing to symbols are used to indicate unknown but anticipated target genes. The right-hand part of the figure lists in bold genes that are transcriptionally upregulated during sporulation in a way that requires one or more of the indicated regulators encoded by whiA, whiB, whiG, whiI, or whiH. Other listed genes are some examples that are upregulated during sporulation, but for which there is no data available regarding the mechanism of their dependence on early whi genes.