Research Interests

Work in my laboratory focuses on morphological differentiation in the filamentous bacterium Streptomyces coelicolor. Although this microbe is best known for its capacity to produce a vast array of secondary metabolites that include antimicrobial, antitumor and immunomodulatory agents, its life cycle is also of interest. Specifically, streptomycete growth starts with the germination of a spore into substrate-penetrating vegetative hyphae. Upon the receipt and processing of environmental signals, the vegetative hyphae begin to differentiate into upwardly growing aerial hyphae. The aerial mycleium serves as the precursor structure for spore formation because as hyphae continue to grow, they curl and form crosswalls that give rise to chains of spores. It is the release of individual spores that disseminates the organism in nature. Indeed, these soil bacteria have a cosmopolitan distribution and generally make up a significant per cent of soil microbiota.

We are particularly interested in the transition between vegetative and aerial hyphae formation. What triggers this differentiation and what are the important structural components that must be produced to allow the filaments to grow into the air? We have demonstrated that intercellular communication appears to be involved in the initiation of differentiation. This can be observed when certain S. coelicolor mutants blocked in aerial hyphae formation (bld mutants) are grown next to one another on rich medium. Under these conditions, nutrient limitation is presumably not a factor in signaling differentiation. Rather, it appears that mutant strains exchange molecular signals a unidirectional fashion such that the strain producing the signal retains its mutant phenotype, while the mutant strain growing nearby undergoes complete morphological differentiation. The pattern of this extracellular complementation allows us to group many of the bld mutants in an hierarchical fashion such that the mutant at one end of the cascade complements all other bld mutants but is complemented by none, while at the other end of the cascade, the bld mutant is complemented by all other bld strains but is incapable of complementing other strains.

What about the structural components involved in the erection of aerial hypae? Further experiments on rich medium have demonstrated that a small hydrophobic peptide, SapB, is temporally and spatially limited to aerial hyphae and spores. Its role in aerial hyphe formation is best demonstrated by the application of purified peptide to bld mutants; such extracellular application to growing hyphae results in their upward growth. We have shown that this peptide functions as a biological detergent, releasing the surface tension so that hyphae can escape the aqueous mileiu of the colony surface and grow up into the air. Surprisingly, fungal hydrophobins, which do not resemble SapB structurally, but are also extremely surface active, can substitute for SapB in the extracellular complementation of bld mutants.

SapB and SapT, a peptide from the species Streptomyces tendae, both function as biosurfactants, and both have an unusual lantibiotic-like structure. Lantibiotics, also known as class I bacteriocins, are a family of ribosomally- synthesized peptide antibiotics that are produced by gram-postiive bacteria. Immediately following translation, these peptides are modified such that specific serine and threonine residues are dehydrated to didehydroalanine and didehydrobutyric acid (Dha and Dhb, respectively). Dha and Dhb then react with nearby cysteine residues to form thioether bonds called lanthionine or methyl lanthionine bridges. Finally, the N terminal lleader sequence is cleaved. SapB and SapT are thus far the only lanthionine-containing peptides that do not function as antibiotics, but rather as morphogenetic proteins. We are interested in how SapB is regulated, how it may interact with other hydrophobic, surface-active proteins called the chaplins, and in probing structure-function relationships.