Figure 7.

Evidence of defective rRNA processing in zmsmu2-1 endosperm. A, Diagram illustrating the maize rRNA transcription unit. Solid bars indicate sequences present in mature 18S, 5.8S, and 25S rRNAs after primary transcript processing. Lines between the bars correspond to the 5′ ETS, ITS1 and 2, and the 3′ ETS. Gray lines below the transcription unit show the location of probes used for northern blots. B, Northern-blot analysis comparing levels of mature and unprocessed rRNAs in wild-type and zmsmu2-1 endosperm. Each lane contained 1 μg of total RNA extracted from genotyped endosperm. +/+, Endosperm from a kernel with a homozygous wild-type embryo; −/−, endosperm from a kernel with a homozygous zmsmu2-1 embryo. Number above each blot indicates the probes used for the northern blot and corresponds to the number in the diagram in A. The blots were exposed to x-ray film for 1 h (blots 2, 4, and 6) or overnight (blots 1, 3, and 5). Four major bands (arrowheads; labeled a, b, c, and d) were detected, and the diagrams on the right show the intermediate and mature rRNA transcripts predicted from the band pattern. Note that bands corresponding to 5.8S rRNA are not shown here due to their small size. C, Northern-blot analysis comparing the ITS2 levels in wild-type, zmsmu2-1, and zmsmu2-3 mutants. The arrowheads indicate unprocessed rRNA species containing ITS2. Each lane contained 1 μg of total RNA. Ethidium bromide-stained 25S and 18S rRNA are shown below as the loading control.