The mitoribosomes

So great an interest in numerous laboratories toward the understanding of the mitoribosome structures and functions comes from at least the two following considerations: a) in the field of the biogenesis of mitochondria, the mitoribosome is a biological key structure, on which synthesis of fundamental mitochondrial elements depends; b) from a phylogenetic point of view questions about the ancestral origin of mitochondrial genomes remain open. Mitoribosomes resemble other ribosomes in their fundamental properties. They are constituted of two subunits containing RNA and proteins. They function according to the same overall mechanism, using initiator tRNA, aminoacyl-tRNA and factors for initiation and elongation to translate mRNA. Mitoribosomes have been observed in situ and their composition has been established by ultrastructural cytochemistry. They are preferentially associated with the inner mitochondrial membrane and are occasionally aligned in "mitopolysomes". Mitoribosomes have been isolated, obtained whole or dissociated into subunits. Frequently, fine morphological details permit to distinguish the mitoribosomes from their cytoplasmic counterparts. The diversity found in various physico-chemical properties (S coefficient, molecular weight, buoyant density, RNA/protein ratio, RNA and protein characteristics) of mitoribosomes indicates that this class of ribosomes is the more heterogeneous. Small and large mrRNA from various organisms showed frequent homologies and conserved basic secondary structures (these similarities depending on the organism) in defined RNA regions, when compared with their counterpart molecules in other ribosomes from various origins. These regions are probably involved in the maintenance of fundamental active conformation. Post-transcriptional oligoadenylation of 3'-termini of the small and large mrRNA in mammals appears to be a general phenomenon. Methylated nucleotides in large and small mrRNA are rare, but their presence seems to constitute an important feature, for they have been phylogenetically conserved and are located in regions of the mrRNA molecules which show a high degree of primary sequence conservation. One unique feature of the mitoribosomes of animal and fungal cells is the absence of 5 S and 5.8 S rRNA molecules. But a well-established exception to the general absence of 5 S RNA is the presence of this RNA molecule in mitoribosomes extracted from higher plants.(ABSTRACT TRUNCATED AT 400 WORDS)