MITOCHONDRIAL-DNA ARRANGED INTO CHROMATIN-LIKE STRUCTURES AFTER INJECTION INTO AMPHIBIAN OOCYTE NUCLEI

Purified mitochondrial DNA (mitDNA) from ovaries of X. laevis was injected into the nuclei (germinal vesicles) of large vitellogenic oocytes of the same organism and examined by EM of the spread nuclear contents. Normally located nuclei of untreated oocytes and peripherally translocated nuclei of centrifuged oocytes were used. Oocyte nuclei isolated and incubated under liquid paraffin oil were injected with DNA. The integrity of transcriptional structures of endogenous chromosomal (lampbrush chromosomes) and extrachromosomal (nucleoli) genes of the injected nuclei was demonstrated. Microinjected mitDNA was identified as circles of chromatin exhibiting polynucleosome-like organization and a mean contour length of 2.6 .mu.m, corresponding to a compaction ratio of the mitDNA of .apprx. 2:1. This DNA packing ratio is similar to that observed after preparation of various kinds of native chromatin in low salt buffers. The chromatin circles formed from injected mitDNA only very rarely exhibited lateral fibrils suggestive of transcriptional activity. Purified mitDNA can be transformed to normally structured chromatin when exposed to oocyte nuclear content but is rarely, if at all, transcribed in this form and in this environment.