Streptomycetes naturally produce a variety of secondary
metabolites, in the process of physiological differentiation.
Streptomyces venezuelae differentiates into spores in liquid
media, serving as a good model system for differentiation and
a host for exogenous gene expression. Here, we report the
growth and differentiation properties of S. venezuelae ATCC-
15439 in liquid medium, which produces pikromycin, along
with genome-wide gene expression profile. Comparison of
growth properties on two media (SPA, MYM) revealed that
the stationary phase cell viability rapidly decreased in SPA.
Submerged spores showed partial resistance to lysozyme and
heat, similar to what has been observed for better-characterized
S. venezuelae ATCC10712, a chloramphenicol producer.
TEM revealed that the differentiated cells in the submerged
culture showed larger cell size, thinner cell wall than
the aerial spores. We analyzed transcriptome profiles of cells
grown in liquid MYM at various growth phases. During
transition and/or stationary phases, many differentiationrelated
genes were well expressed as judged by RNA level,
except some genes forming hydrophobic coats in aerial mycelium.
Since submerged spores showed thin cell wall and
partial resistance to stresses, we examined cellular expression
of MreB protein, an actin-like protein known to be required
for spore wall synthesis in Streptomycetes. In contrast to
aerial spores where MreB was localized in septa and spore
cell wall, submerged spores showed no detectable signal.
Therefore, even though the mreB transcripts are abundant in
liquid medium, its protein level and/or its interaction with
spore wall synthetic complex appear impaired, causing thinner-
walled and less sturdy spores in liquid culture.