The Distribution of Gastrointestinal Parasites in Two Populations of Common Mole-Rats ( Cryptomys hottentotus hottentotus).

The spread of parasites through a host population is based on the variation in behavior and immune function between individuals and is rarely uniform. We studied the gastrointestinal parasites of common mole-rats ( Cryptomys hottentotus hottentotus, Lesson 1826) from 2 sites and assessed the levels of infection based on host sex, breeding status, and season. Only nematode species were found: Neoheligmonella sp. and Mammalakis macrospiculum (Ortlepp, 1939) and a single specimen of Trichuris sp., all of which have direct life cycles. Parasite burden and species richness was greater in the mesic habitat. The abundance of Neoheligmonella sp. differed significantly between seasons, and the season of peak abundance differed between sites, perhaps due to differences in host densities between sites. In addition, parasite burden did not differ between the sexes, but breeding animals had higher infections of Neoheligmonella sp. and M. macrospiculum than non-breeding animals. This and previous studies thus suggest that the subterranean environment is beneficial in reducing parasite diversity, although the restrictions on movement may lead to certain individuals suffering higher parasite burdens.

Analysis of chloroplast transit peptide function using mutations in the carboxyl-terminal region.

Protein import into chloroplasts requires a transit peptide, which interacts with the chloroplast transport apparatus and leads to translocation of the protein across the chloroplast envelope. While the amino acid sequences of many transit peptides are known, functional domains have been difficult to identify. Previous studies suggest that the carboxyl terminus of the transit peptide for ribulose bisphosphate carboxylase small subunit is important for both translocation across the chloroplast envelope and proper processing of the precursor protein. We dissected this region using in vitro mutagenesis, creating a set of mutants with small changes in primary structure predicted to cause alterations in secondary structure. The import behavior of the mutant proteins was assessed using isolated chloroplasts. Our results show that removal of a conserved arginine residue in this region results in impaired processing, but does not necessarily affect import rates. In contrast, substituting amino acids with low reverse turn or amphiphilic potential for other original residues affected import rate but not processing.

Current views on chloroplast protein import and hypotheses on the origin of the transport mechanism.

Most chloroplastic proteins are synthesized as precursors in the cytosol prior to their transport into chloroplasts. These precursors are generally synthesized in a form that is larger than the mature form found inside chloroplasts. The extra amino acids, called transit peptides, are present at the amino terminus. The transit peptide is necessary and sufficient to recognize the chloroplast and induce movement of the attached protein across the envelope membranes. In this review, we discuss the primary and secondary structure of transit peptides, describe what is known about the import process, and present some hypotheses on the evolutionary origin of the import mechanism.

Characterization of a virescent chloroplast mutant of tobacco.

Virescent mutations produce plants in which young leaves have reduced chlorophyll levels but accumulate nearly normal amounts of chlorophyll as they age; they are predominantly nuclear mutations. We describe here a virescent mutation (designated Vir-c) found in a somatic hybrid line derived from Nicotiana tabacum L. and Nicotiana suaveolens Lehm. This mutation is inherited maternally. Young, half-expanded Vir-c leaves contained three to six times less chlorophyll than did control leaves, and reached maximum chlorophyll levels much later in development. Chlorophyll synthesis rates and chloroplast numbers per cell in Vir-c were similar to the control, and carotenoid content in Vir-c was sufficient to protect chlorophyll from photo-oxidation. Photosynthetic rates of Vir-c at low light intensities suggested a reduced ability to collect light. Electron micrographs of Vir-c chloroplasts from half-expanded leaves showed a significant reduction in thylakoids per granum. The decrease in granal thylakoids was strongly associated with low chlorophyll levels; mature Vir-c leaves with nearly normal chlorophyll content showed normal granal profiles. These results are discussed in relation to virescent mutants previously described.

The Phenotype of a Virescent Chloroplast Mutation in Tobacco Is Associated with the Absence of a 37.5 kD Thylakoid Polypeptide.

The Vir-c mutation is a virescent chloroplast mutation found in a line of plants derived from protoplast fusions between a Nicotina tabacum line and a line containing N. tabacum nuclei with Nicotiana suaveolens cytoplasm. Vir-c displays a lag period in chlorophyll accumulation and granal stack formation in young leaves. We examined total chloroplast protein in young leaves and showed the mutant contains 1.3 to 2.1 times less stromal protein, and 2.9 to 4.3 times less thylakoid protein when compared to the N. tabacum var "Turkish Samsun" control. Electrophoretic patterns of total thylakoid proteins indicated three polypeptides were specifically decreased in amount within the context of the overall reduction in thylakoid protein. Electrophoresis of thylakoid proteins synthesized by chloroplasts isolated from half-expanded leaves demonstrated that mutant chloroplasts did not synthesize a 37.5 kilodalton polypeptide which was synthesized by "Samsun" chloroplasts. A polypeptide of this molecular weight was synthesized by Vir-c chloroplasts isolated from mature leaves which had recovered the normal phenotype. Restriction digestion and electrophoresis of the mutant's chloroplast DNA produced a pattern of restriction fragments different from either N. tabacum or N. suaveolens chloroplast DNA.