The initiation of development involves both sensing of nutritional stimuli and complex extracellular signalling, including quorum sensing, extracellular proteases, and other putative signals (see e.g. [3–5]). The formation of aerial hyphae depends on a series of mostly regulatory genes that have been designated bld since they are required for the emergence of the hairy aerial mycelium on the colony surface. The regulatory networks governed by these genes are only partially understood, but are gradually being revealed [4, 6, 7]. check details The subsequent

development of the aerial hyphae into spores can be blocked at different stages by mutating critical genes. Many mutations of this type give rise to a white aerial mycelium due to a failure to produce the grey spore pigment. Isolation of such whi mutants was the basis for identifying central regulatory genes that direct sporulation in aerial hyphae (for recent reviews, see [1, 4]). A major challenge in Streptomyces developmental biology is now to Selleckchem BGB324 decipher how these regulators are acting to control the physiological and cell cycle-related processes involved in producing the mature spores, including modulation of cell division, cell wall assembly, chromosome replication, and nucleoid partitioning and condensation. The accompanying physiological responses include for example the cell type-specific

accumulation and utilisation of glycogen and trehalose, and the synthesis of a polyketide spore pigment. The biosynthetic genes for the

pigment are found in the whiE gene cluster, and the expression of this cluster depends on the regulatory whi genes, although the direct regulator is still unknown [8, 9]. The identified regulatory whi genes that are required for the early stages of sporulation in aerial hyphae appear to fall into two major and converging pathways [1]. The RNA polymerase sigma factor σWhiG is required for the initiation of spore 2-hydroxyphytanoyl-CoA lyase formation in S. coelicolor and controls two other regulatory genes, whiI encoding a response regulator and whiH encoding a GntR-family protein [10–13]. Genetic analyses show that whiG mutations block progression of differentiation at an early stage of apparently undifferentiated aerial hyphae in S. coelicolor, and whiG mutations are epistatic on both whiI and whiH[14, 15]. The phenotypes of whiI and whiH mutants differ in that whiI mutants do not form sporulation septa and do not show pronounced nucleoid condensation, while whiH mutants are able to convert the apical cells of some aerial hyphae into spore-like fragments with condensed nucleoids and occasional sporulation septa [12, 13, 15]. WhiH is autoregulatory and binds to its own promoter region [16], while WhiI (C-terminal fragment) binds to one independent target promoter (for inoRA) [17, 18]. However, no other direct targets for WhiH or WhiI have been reported.