Fig. 7. Formation of mini-CBs in the absence of U7 snRNA. Confocal images of CBs stained with mAb K 121 against the trimethylguanosine cap (first column) and counterstained with -coilin serum C236 to identify the mini-CBs (second column). The overlay is shown in the third column and a DIC image is shown in the fourth column. (A,B) Mini-CB (arrowheads in A) and typical CB (cb in B) from an oocyte that was heat shocked at 31.5°C for 4 hours and allowed to recover overnight. The mini-CB and the typical CB stain strongly for both trimethylguanosine (green) and coilin (red). The mini-CB is closely associated with a B-snurposome (b). Nucleolus (n). (C) Mini-CB (arrowheads) and typical CB (cb) from an oocyte that was heat shocked after injection of an oligodeoxynucleotide against U7 snRNA. Elimination of U7 causes a marked reduction in trimethylguanosine staining of CBs, because U7 is the major capped snRNA in CBs. Mini-CBs are induced by heat shock in U7-depleted oocytes; they too are deficient in trimethylguanosine staining (arrowheads in C). Neither the morphology nor coilin staining of CBs is affected by U7 depletion.