RNA synthesis in isolated mitochondria from sea urchin embryos

Silencing of maternal heme-binding protein causes embryonic mitochondrial dysfunction and impairs embryogenesis in the blood sucking insect Rhodnius prolixus

The heme molecule is the prosthetic group of many hemeproteins involved in essential physiological processes, such as electron transfer, transport of gases, signal transduction, and gene expression modulation. However, heme is a pro-oxidant molecule capable of propagating reactions leading to the generation of reactive oxygen species. The blood-feeding insect Rhodnius prolixus releases enormous amounts of heme during host blood digestion in the midgut lumen when it is exposed to a physiological oxidative challenge. Additionally, this organism produces a hemolymphatic heme-binding protein (RHBP) that transports heme to pericardial cells for detoxification and to growing oocytes for yolk granules and as a source of heme for embryo development. Here, we show that silencing of RHBP expression in female fat bodies reduced total RHBP circulating in the hemolymph, promoting oxidative damage to hemolymphatic proteins. Moreover, RHBP knockdown did not cause reduction in oviposition but led to the production of heme-depleted eggs (white eggs). A lack of RHBP did not alter oocyte fecundation. However, produced white eggs were nonviable. Embryo development cellularization and vitellin yolk protein degradation, processes that normally occur in early stages of embryogenesis, were compromised in white eggs. Total cytochrome c content, cytochrome c oxidase activity, citrate synthase activity, and oxygen consumption, parameters that indicate mitochondrial function, were significantly reduced in white eggs compared with normal dark red eggs. Our results showed that reduction of heme transport from females to growing oocytes by RHBP leads to embryonic mitochondrial dysfunction and impaired embryogenesis.

Identification of two homologous mitochondrial DNA sequences, which bind strongly and specifically to a mitochondrial protein of Paracentrotus lividus

Using a combination of band shift and DNasel protection experiments, two Paracentrotus lividus mitochondrial sequences, able to bind tightly and selectively to a mitochondrial protein from sea urchin embryos, have been found. The two sequences, which compete with each other for binding to the protein, are located in two genome regions which are thought to contain regulatory signals for mitochondrial replication and transcription. A computer analysis suggests that the sequence TTTTRTANNTCYYATCAYA, common to the two binding regions, is the minimal recognition signal for the binding to the protein. We discuss the hypothesis that the protein binding capacity of these two sequences is involved in the control of sea urchin mtDNA replication during developmental stages.

The cytochrome b of the sea urchin Paracentrotus lividus is naturally resistant to myxothiazol and mucidin

The ubiquinol:cytochrome c reductase activity of Paracentrotus lividus mitochondria is relatively insensitive to the specific inhibitors myxothiazol and mucidin. The I50 of myxothiazol and mucidin are three and two orders of magnitude higher, respectively, in P. lividus than in bovine heart mitochondria. The natural resistance of the P. lividus reductase to these inhibitors can be correlated with a single amino replacement, an alanine for a glycine at position 143, in the sequence of cytochrome b. This position is located in a conserved region of the molecule, believed to be important in the oxidation of ubiquinol by the reductase.

Mapping and characterization of Paracentrotus lividus mitochondrial transcripts: multiple and overlapping transcription units

This paper reports the mapping of both mature and precursor Paracentrotus lividus mitochondrial transcripts. Several mtRNAs were found to have 5' and 3' termini which differ from those inferred through DNA sequencing (Cantatore et al. 1989). The 3' ends of the two rRNAs (12S and 16S) overlap with the downstream transcripts (tRNAGlu and CoI mRNA) by 5 and 10 nt respectively. The 132 nt non-coding region is extensively transcribed: in particular it contains a 124 nt RNA and the 5' end of a possible precursor of 13 clustered tRNAs. This latter overlaps by 7 nt with the 3' end of the 124 nt RNA. In addition to the mature RNAs, 32 high molecular weight RNAs, which are probably the precursors of the smaller more abundant mature species, were detected by Northern blotting. The mapping of these transcripts indicates that they are processed at the level of tRNA or tRNA-like sequences and suggests the existence of two transcription initiation sites upstream of the ND1 and the cytochrome b genes respectively. In the light of these results it appears that P. lividus mitochondrial DNA transcription takes place via multiple and probably overlapping transcription units. Moreover, the wide variation in the steady-state levels of the mature mRNAs indicates that sea urchin mitochondrial DNA expression is also regulated at the level of RNA decay.

Determination of some mitochondrial RNAs concentration in adult rat liver

A method has been developed to accurately measure steady state concentrations of mitochondrial transcripts in adult rat liver. Total mitochondrial RNA has been hybridized with an excess of labelled mitochondrial DNA fragments coding for a single gene or a piece of it. The results obtained show that each mitochondrion contains 36 molecules of 16S rRNA, 81 of 12S rRNA and about 8 molecules of mRNAs coding for identified and unidentified reading frames. Thus, the rRNA/mRNA ratio in rat liver differs from that reported in HeLa cell mitochondria. These results are discussed in the light of mitochondrial DNA transcription regulation.