Nucleic Acids Research, 2008, Vol. 36, No. 22 7247 Figure 4. sRNAs exhibiting altered expression proﬁles in bld or whi developmental mutants. (A) Top panels: comparison of scr1906 expression in bldA and bldB mutants relative to wild-type on rich medium (RM); bottom panels: scr1906 expression in wild-type versus whiG mutant on minimal medium (MM). (B) scr3558 expression in a bldA mutant (left panels), and a bldB mutant (right panels) during growth on RM and MM. (C) Expression of scr3974 in a bldB mutant (right panel) relative to wild-type strain (left panel) on RM. (D) Comparison of scr4677 expression in a wild- type strain (left panel) and a bldA mutant strain (right panel) on RM. (E) scr2101 expression in wild-type, bldA and whiG mutants during growth on RM (top panels), and expression in wild-type relative to a bldB mutant after culturing on MM (bottom panels). To ensure equivalent comparisons, wild-type and mutant RNA samples were transferred to the same membrane after separation by PAGE, and thus were subjected to identical hybridization and washing conditions, and were exposed to X-ray ﬁlm for the same period of time. Time of RNA harvest (post-inoculation) is indicated on the top of each sRNA northern blot. 5S rRNA served as a control for RNA loading and RNA integrity. Northern blot analysis of wild- type and mutant RNA samples were repeated at least three times with independent RNA samples. microarray analysis for 24 of these (apart from 5S RNA and tmRNA, they could not detect any transcripts using northern blotting) (39). Two of the predicted 32 sRNAs overlapped with sRNAs identiﬁed here: scr3974 and scr3558. Features of identified sRNAs in S. coelicolor sRNA expression was examined under two distinct culture conditions—rich and minimal solid media. The expres- sion of most sRNAs showed some degree of medium- speciﬁcity, with all but scr2101 and scr3558 being