Different cellular backgrounds confer a marked advantage to either mutant or wild-type mitochondrial genomes.

After the introduction of mitochondria with a mixture of mutant and wild-type mitochondrial DNA (mtDNA) into a human rho degree cell line (143B.206), Yoneda et al. [Yoneda, M., Chomyn, A., Martinuzzi, A., Hurko, O. & Attardi, G. (1992) Proc. Natl. Acad. Sci. USA 89, 11164-11168] observed a shift in the proportion of the two mitochondrial genotypes in a number of cybrid clones. In every case where a shift was observed, there was an increase in the proportion of mutant mtDNA. By using the same cell line (143B.206 rho degree), we also generated cybrids that were either stable in their mitochondrial genotype or showed an increase in the proportion of mutant mtDNA. However, temporal analysis of the same mutant mtDNA type in another rho degree cell line revealed a quite distinct outcome. Those clones that showed a change shifted toward higher levels of wild-type rather than mutant mtDNA. These results indicate that the nuclear genetic background of the recipient (rho degree) cell can influence the segregation of mutant and wild-type mitochondrial genomes in cell cybrids.

Human steroidogenic acute regulatory protein: functional activity in COS-1 cells, tissue-specific expression, and mapping of the structural gene to 8p11.2 and a pseudogene to chromosome 13.

Steroidogenic acute regulatory protein (StAR) appears to mediate the rapid increase in pregnenolone synthesis stimulated by tropic hormones. cDNAs encoding StAR were isolated from a human adrenal cortex library. Human StAR, coexpressed in COS-1 cells with cytochrome P450scc and adrenodoxin, increased pregnenolone synthesis > 4-fold. A major StAR transcript of 1.6 kb and less abundant transcripts of 4.4 and 7.5 kb were detected in ovary and testis. Kidney had a lower amount of the 1.6-kb message. StAR mRNA was not detected in other tissues including placenta. Treatment of granulosa cells with 8-bromo-adenosine 3',5'-cyclic monophosphate for 24 hr increased StAR mRNA 3-fold or more. The structural gene encoding StAR was mapped using somatic cell hybrid mapping panels to chromosome 8p. Fluorescence in situ hybridization placed the StAR locus in the region 8p11.2. A StAR pseudogene was mapped to chromosome 13. We conclude that StAR expression is restricted to tissues that carry out mitochondrial sterol oxidations subject to acute regulation by cAMP and that StAR mRNA levels are regulated by cAMP.