Alterations of pre-rRNA processing in skin fibroblasts from DBA patients. (A) Northern blot analysis of total RNA extracts from control and DBA fibroblasts with the 5′-ITS1 probe. The β-actin mRNA is detected as a loading control. (B) Intensity profiles of the hybridization signal in each lane (phosphoimaging) normalized to the level of actin mRNA. (C) Variation of the relative levels of 21S and 18S-E pre-rRNAs as compared with control cells. The amounts of the 2 species were measured after Northern blotting with the 5′-ITS1 probe by phosphoimaging. The results were analyzed with the Student t test. Error bars indicate standard deviation; n, number of experiments. (D) Pulse-chase analysis of pre-rRNA processing with ³H-methylmethionine. The cells were labeled for 30 minutes with ³H-methylmethionine, and chase was performed for the indicated time. Contrast in the panel corresponding to MUTS-A cells was enhanced by image processing (“enhanced”). All other panels correspond to identical exposure and processing. The 28S rRNA is shown as a loading control.