Nucleic Acids Research, 2008, Vol. 36, No. 22 7245 Figure 2. Experimental veriﬁcation of sRNAs identiﬁed using a comparative genomic search of intergenic regions. Panels on the right indicate the positioning of each sRNA (shown in black) relative to adjacent protein-coding genes (shown in grey), whilst panels on the left show northern blots probed with labelled DNA oligonucleotides (together with a 25 basepair ladder shown adjacent to scr1906). For northern blot analysis, RNA was harvested from S. coelicolor grown on either minimal or rich medium at four or ﬁve diﬀerent timepoints (indicated as hours post-inoculation above each sRNA blot). 5S rRNA was used as a positive control for RNA loading and RNA integrity. For each sRNA, northern blot analysis was carried out using at least three diﬀerent RNA samples to ensure the reproducibility of expression proﬁles. bld mutants are impaired in their ability to raise aerial hyphae, and in many instances are also defective in anti- biotic production, while whi mutants are blocked during the maturation of aerial hyphae and fail to form mature spores (3). We examined sRNA expression in two bld (bldA and bldB) and whi (whiG and whiB) mutant strains. bldA and bldB mutants share a defect in antibiotic produc- tion, but have distinct developmental phenotypes: the bldA mutant is a conditional mutant whose inability to raise aerial hyphae can be rescued by growth on minimal medium with mannitol, whilst bldB has a non-conditional mutant phenotype and consequently is defective in aerial hyphae formation under all growth conditions (37). In contrast, whiB and whiG disruption strains have similar mutant phenotypes in that both mutants are blocked early in the sporulation process; however, the whiB and whiG gene products are predicted to act in diﬀerent pathways leading to mature spore formation (the function of WhiB has yet to be deﬁnitively determined, whilst WhiG acts as a sporulation-speciﬁc sigma factor) (38). The bld and whi mutations had no eﬀect on the trans- cription of four sRNAs: scr3045, scr3261, 3287 and scr5676. Expression of the remaining sRNAs was aﬀected to varying degrees, with scr1906 in particular exhibiting a dramatically altered transcription proﬁle. In a wild-type genetic background, scr1906 was expressed during aerial