Bioassays of nutrient limitation in a tropical rain forest soil

Summary Six speices of shrubs and one large herb with contrasting life history patterns were used as bioassays of nutrient availability in a Costa Rican lowland rain forest soil. Growth responses of the herb (Phytolacca rivinoides, Phytolaccaceae) confirmed soil measurements indicating high availability of N and potentially limiting levels of P, K, Mg and Ca. Growth responses of the shrub species (Miconia spp., Melastomataceae and Piper spp., Piperaceae) to a complete nutrient fertilizer were generally less than that of Phytolacca. Lack of a strong shrub response to +P fertilization is probably due to mycorrhizal associations and slower growth rates of woody species. In general, increased growth did not occur at the expense of phenolic production in the leaves. The results emphasize that assessment of specific nutrient limitations to plant growth vary depending on species selected for the bioassay, even among species from the same community.     

Mechanism of mRNA binding to bovine mitochondrial ribosomes

The binding of mRNA to bovine mitochondrial ribosomes was investigated using triplet codons, homopolymers and heteropolymers of various lengths, and human mitochondrial mRNAs. In the absence of initiation factors and initiator tRNA, mitochondrial ribosomes do not bind triplet codons (AUG and UUU) or homopolymers (oligo(U] shorter than about 10 nucleotides. The RNA binding domain on the 28 S mitoribosomal subunit spans approximately 80 nucleotides of the mRNA, judging from the size of the fragments of poly(U,G) and natural mRNAs protected from RNase T1 digestion by this subunit, but the major binding interaction with the ribosome appears to occur over a 30-nucleotide stretch. Human mitochondrial mRNAs coding for subunits II and III of cytochrome c oxidase and subunit 1 of the NADH-ubiquinone oxidoreductase (complex I) were used in studying in detail the binding of mRNA to the small subunit of bovine mitochondrial ribosomes. We have determined that these mRNAs have considerable secondary structure in their 5'-terminal regions and that the initiation codon of each mRNA is sequestered in a stem structure. Little mRNA was bound to ribosomes in a manner conferring protection of the 5' termini from RNase T1 digestion, under standard conditions supporting the binding of artificial templates, but such binding was greatly stimulated by the addition of a mitochondrial extract. Initiation factors and tRNAs from Escherichia coli were unable to stimulate the 5' terminus protected binding of these mRNA molecules, demonstrating a requirement for homologous factors. Our results strongly suggest that mitochondrial initiation factors are required for the proper recognition and melting of the secondary structure in the 5'-terminal region of mitochondrial mRNAs, as a prerequisite for initiation of protein synthesis in mammalian mitochondria.     

Patterns of plant species diversity during succession under different disturbance regimes

Summary I suggest that between-community variations in diversity patterns during succession in plant communities are due to the effects of selection on life history strategies under different disturbance regimes. Natural disturbances to plant communities are simultaneously a source of mortality for some individuals and a source of establishment sites for others. The plant community consists of a mosaic of disturbance patches (gaps) of different environmental conditions. The composition of the mosaic is described by the size-frequency distribution of the gaps and is dependent on the rates and scales of disturbance. The life-history strategies of plant species dependent on some form of disturbance for establishment of propagules should reflect this size-frequency distribution of disturbance patches. An extension of island biogeographic theory to encompass relative habitat area predicts that a community should be most rich in species adapted to growth and establishment in the spatially most common patch types. Changes in species diversity during succession following large scale disturbance reflect the prevalent life history patterns under historically common disturbance regimes. Communities in which the greatest patch area is in large-scale clearings (e.g. following fire) are most diverse in species establishing seedlings in xeric, high light conditions. Species diversity decreases during succession. Communities in which such large patches are rare are characterized by a large number of species that reach the canopy through small gaps and realtively few which regenerate in the large clearings. Diversity increases during succession following a large scale disturbance.Evidence from communities characterized by different disturbance regimes is summarized from the literature. This hypothesis provides an evolutionary mechanism with which to examine the changes in plant community structure during succession. Diversity peaks occurring at intermediate levels of disturbance as discussed by Connell and Huston are interpreted in this context.     

Mammalian mitochondrial ribosomal proteins (2). Amino acid sequencing, characterization, and identification of corresponding gene sequences

Four different classes of mammalian mitochondrial ribosomal proteins were identified and characterized. Mature proteins were purified from bovine liver and subjected to N-terminal or matrix-assisted laser-desorption mass spectroscopic amino acid sequencing after tryptic in-gel digestion and high pressure liquid chromatography separation of the resulting peptides. Peptide sequences obtained were used to virtually screen expressed sequence tag data bases from human, mouse, and rat. Consensus cDNAs were assembled in silico from various expressed sequence tag sequences identified. Deduced mammalian protein sequences were characterized and compared with ribosomal protein sequences of Escherichia coli and yeast mitochondria. Significant sequence similarities to ribosomal proteins of other sources were detected for three out of four different mammalian protein classes determined. However, the sequence conservation between mitochondrial ribosomal proteins of mammalian and yeast origin is much less than the sequence conservation between cytoplasmic ribosomal proteins of the same species. In particular, this is shown for the mammalian counterparts of the E. coli EcoL2 ribosomal protein (MRP-L14), that do not conserve the specific and functional highly important His(229) residue of E. coli and the corresponding yeast mitochondrial Rml2p.