Characterization of regulatory pathways controlling morphological differentiation in "Streptomyces coelicolor"

Abstract

The filamentous eubacterium, Streptomyces coelicolor, undergoes a complex cycle of growth and development in which morphological differentiation coincides with the activation of the orphan response regulator RamR and the biosynthesis of a morphogenic peptide called SapB. SapB is a lantibiotic-like molecule derived from the product of the ramS gene that promotes aerial hyphae formation (AHF) by breaking the aqueous tension on the surface of the substrate mycelium. A ramR-disrupted mutant is delayed in AHF, while constitutive overexpression of ramR accelerates AHF in the wild type strain and restores SapB biosynthesis and AHF in all of the developmental mutants (bld) tested. Here we report DNA array analyses suggesting the identity of more than 300 genes belonging to the ramR regulon, leading to the discovery of a locus of cotranscribed developmental genes that has a role in both AHF and sporulation. The genes of this cluster of ramR dependent genes (rdg) include: rdgA (sco4075) and rdgB (sco4074), which encode two subunits of an ABC transporter, rdgK (sco4073), a putative histidine kinase, and rdgR (sco4072), a ramR paralog. Promoter mapping and protein-DNA binding experiments indicate that RamR activates rdgABKR transcription directly, by binding to three sequence motifs in the rdgABKR promoter region. A constructed rdgABKR null mutant was able to synthesize SapB and erect aerial hyphae, however these hyphae were unusually branched, reminiscent of substrate hyphae. Subsequent stages of differentiation, septation and sporogenesis were delayed. The role of rdgABKR in AHF was shown both by epistasis (rdgR activated AHF in bld mutants) and extracellular complementation (rdgR induced synthesis of an activity allowing AHF in bld mutants) experiments. In addition, using microarray and quantitative real time RT-PCR analyses we found that initiation of aerial growth, induced by SapB, provoked a rapid increase in the transcriptional abundance of morphogenetic genes, such as rdlAB and chpABDFG. A slower response and lower levels of induction were also observed for chpH, chpC. The expression of chpE was weakly stimulated by SapB. In addition to rdlAB and chpABCDFGH, transcript abundance of previously uncharacterized genes, encoding either hydrophobic proteins (sco0323, sco1076, sco2703, sco4002, sco4902), enzymes involved in cell wall biosynthesis (sco2983), or in production of terpene compounds (sco7700 and sco7701) was also significantly upregulated betweenh andh of aerial growth. Conversely, transcriptional levels of genes possibly involved in the regulation of the energy metabolism (narG2H2, sco0179) or encoding putative transcription factors (sco0166, sco0168) were rapidly downregulated. These results demonstrated that SapB can trigger the activation of a morpgohenetic program to the emerging aerial hyphae.