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Cell differentiation

The ability of some bacteria to differentiate into distinct cell types is of large significance in several aspects of microbiology. Detailed analyses of the developmental programmes in e.g. Bacillus and Caulobacter have also fuelled much progress in molecular cell biology of bacteria. These systems belong today to the best-understood developmental systems of all kinds, and are attractive models in systems biology.

Developmental biology of Streptomyces

The developmental life cycle of streptomycetes belong to the most complex among prokaryotes. A vegetative mycelium of branched hyphae forms a colony, in which coordinated multicellular development leads both to production of antibiotics and the emergence of a specialized aerial mycelium on the colony surface (Fig. 1). Once the aerial hyphae are erected, a second developmental programme - the subject of our investigations - leads to striking re-regulations of growth, cell cycle and morphogenetic processes, and the conversion of the long multigenomic aerial hyphae to chains of prespores. These are further matured to single spores, which are the means for dispersal and survival of these organisms. A spore can eventually germinate and grow out into new mycelium when encountering favourable conditions.

Developmental life cycle of Streptomyces coelicolor

The developmental life cycle of Streptomyces coelicolor. Schematic drawing by Markus Hempel, adapted from originals by K. F. Chater.

Regulation of sporulation

Streptomyces sporulation has a lot to tell about fundamental cell cycle and morphogenetic processes and their regulation. Through elegant genetic and molecular analyses, a series of central regulators of development have been identified (Flärdh and Buttner, 2009) However, relatively few sporulation genes are known, and the underlying regulatory pathways are only partially understood. New technologies and genome sequencing are changing this situation, and we anticipate rapid progress in the understanding of this intriguing developmental system and its interplay with fundamental bacterial cell biology.

Compartmentalised gene expression during Streptomyces sporulation.

Compartmentalised gene expression during Streptomyces sporulation. The promoter of a new sporulation gene, smeAp, was fused to the gene for the red fluorescent protein mCherry. This reveals specific expression of this gene in the hyphal compartment that is converted to spores.

New sporulation genes in Streptomyces

We are using a combination of functional genomic tools, molecular genetics, and cell biological approaches to investigate the developmental programme that converts aerial hyphae into dormant spores. By studying gene expression using DNA microarrays, we have identified a range of new sporulation genes in Streptomyces coelicolor, and are currently analyzing them. They include an operon encoding a small membrane protein, SmeA, and an FtsK/SpoIIIE homologue, SffA, which have pleiotropic effects on spore maturation (Fig. 2; Ausmees et al., 2007). We are also analyzing the role of the WhiH regulator, which has a specific effect on the developmental control of cell division (Ryding et al., 1998).

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