We have identified stable transcripts from the so-called nontranscribed spacer region (NTS) of the nuclear ribosomal DNA repeat in certain respiration-deficient strains of Saccharomyces cerevisiae. These RNAs, which are transcribed from the same strand as is the 37S rRNA precursor, are 500 to 800 nucleotides long and extend from the 5' end of the 5S rRNA gene to three major termination sites about 1,780, 1,830, and 1,870 nucleotides from the 3' end of the 26S rRNA gene. A survey of various wild-type and respiration-deficient strains showed that NTS transcript abundance depended on the mitochondrial genotype and a single codominant nuclear locus. In strains with that nuclear determinant, NTS transcripts were barely detected in [rho'] cells, were slightly more abundant in various mit-derivatives, and were most abundant in petites. However, in one petite that was hypersuppressive and contained a putative origin of replication (ori5) within its 757-base-pair mitochondrial genome, NTS transcripts were no more abundant than in [rho'] cells. The property of low NTS transcript abundance in the hypersuppressive petite was unstable, and spontaneous segregants that contained NTS transcripts as abundant as in the other petites examined could be obtained. Thus, respiration deficiency per se is not the major factor contributing to the accumulation of these unusual RNAs. Unlike RNA polymerase I transcripts, the abundant NTS RNAs were glucose repressible, fractionated as poly(A)+ RNAs, and were sensitive to inhibition by 10 ,ug of a-amanitin per ml, a concentration that had no effect on rRNA synthesis. Abundant NTS RNAs are therefore most likely derived by polymerase H transcription.Although the products of many nuclear genes and a few encoded by the mitochondrial genome are required for the synthesis of functional mitochondria, the molecular interactions between these organelles, which are required to achieve balanced mitochondrial synthesis, are still poorly understood. Expression of some nuclear genes encoding mitochondrial proteins is known to be regulated according to growth conditions and to various metabolic demands of the cell. In the yeast Saccharomyces cerevisiae, for example, growth of cells on high glucose concentrations represses the synthesis of nuclear-encoded components of the oxidative phosphorylation apparatus (55,62). Yeast cells also have a regulatory network that adjusts the synthesis of mitochondrial heme proteins in response to fluctuations in the levels of heme (21). These regulatory pathways operate through interactions between trans-acting factors encoded at loci such as HAP], HAP2, and HAP3 and their target, cis-acting sites located upstream of the relevant genes (22, 44 46, 58).Other nuclear genes have been identified which control mitochondrial gene expression in that their products exert specific control over mitochondrial RNA processing (3,13,14), mRNA stability (13-15), and translation (11,47,52 that the functional state of mitochondria, or the mitochondrial genotype, could influence the expre...
We have identified a path in yeast, from mitochondria to the nucleus, which may have a regulatory function in mitochondrial biogenesis. This path is evident as an elevated expression of a number of nuclear DNA sequences in response to specific defects in the mitochondrial genome, including the absence of mitochondrial DNA in rho 0 petites. Among those nuclear sequences preferentially expressed in certain respiratory-deficient cells are stable poly(A)+ transcripts derived from the so-called non-transcribed spacer region of the nuclear ribosomal DNA repeat, where they are most abundant in the rho 0 petite. Although the function of these unusual RNAs is unclear, the observations may reflect the presence of a mitochondrial homeostatic control system in yeast, which we suggest could function to adjust the mass of mitochondria and mitochondrial DNA in the cell in response to inequities in organelle apportionment during cell budding.
Epithelial cells cultured on type I collagen gels adopt a typical apical—basal polarity and undergo differentiation. We have compared the behaviour of chick embryo retinal pigmented epithelial (RPE) cells on collagen and on plastic with and without gelatin coats. RPE cell proliferation was similar on all three substrata, and post-confluent cultures exhibited multilayering. On plastic and gelatin-coated plastic, dome formation, typical of transporting epithelia, occurred. On type I collagen gels, however, dome formation did not occur, but rather invasion of the gel matrix by cords of epithelial cells took place. In contrast, invasive behaviour of the cells was markedly reduced on type IV coated collagen gels, particularly in the presence of laminin. These results illustrate the prominent role of the extracellular matrix on phenotypic expression by RPE cells and may represent a more general phenomenon.
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