Identification of a 4.5S-like ribonucleoprotein in maize mitochondria

Signal recognition particles in chloroplasts, bacteria, yeast and mammals (Review)

The Signal Recognition Particle (SRP) plays a critical role in the sorting of nascent secretory and membrane proteins. Remarkably, this function has been conserved from bacteria, where SRP delivers proteins to the inner membrane, through to eukaryotes, where SRP is required for targeting of proteins to the endoplasmic reticulum. This review focuses on present understanding of SRP structure and function and the relationship between the two. Furthermore, the similarities and differences in the structure, function and cellular role of SRP in bacteria, chloroplasts, fungi and mammals will be stressed.

The mitochondrial genome of Arabidopsis is composed of both native and immigrant information

Plants contain large mitochondrial genomes, which are several times as complex as those in animals, fungi or algae. However, genome size is not correlated with information content. The mitochondrial genome (mtDNA) of Arabidopsis specifies only 58 genes in 367 kb, whereas the 184 kb mtDNA in the liverwort Marchantia polymorpha codes for 66 genes, and the 58 kb genome in the green alga Prototheca wickerhamii encodes 63 genes. In Arabidopsis' mtDNA, genes for subunits of complex II, for several ribosomal proteins and for 16 tRNAs are missing, some of which have been transferred recently to the nuclear genome. Numerous integrated fragments originate from alien genomes, including 16 sequence stretches of plastid origin, 41 fragments of nuclear (retro)transposons and two fragments of fungal viruses. These immigrant sequences suggest that the large size of plant mitochondrial genomes is caused by secondary expansion as a result of integration and propagation, and is thus a derived trait established during the evolution of land plants.

Depletion of Escherichia coli 4.5S RNA leads to an increase in the amount of protein elongation factor EF-G associated with ribosomes

In Escherichia coli, 4.5S RNA is found in complexes with both protein translocation protein, Ffh (a bacterial homolog of mammalian SRP54) and protein synthesis elongation factor G (EF-G). To analyze the function of 4.5S RNA in translation, we initially assessed the sensitivity of the association of 4.5S RNA with the ribosome after treatment with antibiotics that affect various stages of protein synthesis. Fusidic acid and viomycin caused 4.5S RNA to cosediment with the 70S ribosomal fraction, indicating that 4.5S RNA enters the ribosome before ribosomal translocation and release of EF-G-GDP from the ribosome. On the other hand, depletion of 4.5S RNA led to the retention of a significant amount of EF-G on 70S ribosomes. In addition, 4.5S RNA shares a conserved decanucleotide sequence (58GAAGCAGCCA67) motif with the characterized EF-G-binding site at positions 1068-1077 on 23S RNA. We therefore examined by gel mobility-shift assay whether or not mutations in the domain-IV region of 4.5S RNA, including this conserved motif, disturb the binding of EF-G to 23S RNA. Any mutation at the C62, G64 or A67 residues within this motif abolished competition activity. Therefore, we propose that 4.5S RNA is concerned with the mode of association of EF-G with the ribosomes. Moreover, this function depends on the secondary structure of 4.5S RNA as well as a ten-base sequence conserved between the two RNAs.