Control of Protein Synthesis in Acanthamoeba castellanii

Structure and expression of a cyst specific protein of Acanthamoeba castellanii

The life cycle of Acanthamoeba is divided into a growth-division phase and two distinctive processes of cellular differentiation, termed encystment and excystment. Polyacrylamide gel electrophoresis revealed that a specific protein of 21 kDa in molecular weight occurs in the cyst, but not in the trophozoite stages of A. castellanii Neff strain. This cyst-specific protein, designated as CSP21, was purified from guanidine-HCl extract of cyst wall and anti-CSP21 antibody was produced. Immunoblotting of proteins extracted from a variety of species of Acanthamoeba genus suggested that the antibody is specific for group II amoebae, therefore, providing a useful tool for Acanthamoebae taxonomy. A cDNA clone for A. castellanii CSP21 was isolated by immunoscreening of a cDNA expression library constructed from mRNA of amoebae at encysting stage. The deduced primary structure indicated that CSP21 is a hydrophilic protein showing no significant homology with peptides thus far published. RNA blot analysis showed that the expression of CSP21 mRNA was restricted within early stages of encystment, suggesting that the biosynthesis of CSP21 is regulated at mRNA level.

Effect of essential amino acids on the phosphorylation of a 40S ribosomal protein and protein synthesis in Acanthamoeba castellanii

Reversible and multiple phosphorylation of a 40S ribosomal protein is observed in a variety of eukaryotic cells. In the primitive eukaryote Acanthamoeba, one or three phosphorylated S3 derivatives are observed during growth phase in nondefined nutrient medium (ND cells) or in chemically defined nutrient medium (D cells), respectively. In both cases, stationary phase cells exhibit nonphosphorylated S3; however, transfer of these cells into the respective fresh nutrient media results in a transient accumulation of four phosphorylated S3 derivatives. Transfer of D cells into nutrient medium, deficient in all or any single essential amino acids, leads to reversible inhibition of S3 phosphorylation and growth arrest. The low level of phosphorylated S3 is not simply the consequence of growth arrest, since in cells where growth is arrested differently, the level of phosphorylated S3 can be high. In response to amino acid deficiency, a number of other changes can be observed. These include a 2-3-fold decrease of total protein synthesis, 13 changes in the cellular protein pattern, and specific alterations in the ribosome absorbance profiles and in the distribution of poly-A+ RNA within subribosomal and ribosomal fractions. While the rate of total protein synthesis seems to be associated with the level of phosphorylated S3, the level of the synthesis of at least 10 of the particular proteins can be dissociated from the level of S3 phosphorylation.

Regulation of ribosomal RNA transcription during differentiation of Acanthamoeba castellanii: a review

During the cellular differentiation induced by starvation of Acanthamoeba castellanii, the expression of a number of genes is regulated. Evidence is reviewed that at least one of these, the precursor ribosomal RNA transcription unit, is regulated at the level of transcription. The structure of the rRNA transcription unit and of the RNA polymerases responsible for transcription in Acanthamoeba are reviewed. Utilizing an in vitro transcription system constructed from these components, preliminary evidence has been obtained that pre-rRNA gene expression is regulated by a modification of RNA polymerase I that affects the enzyme's ability to participate efficiently in the initiation of transcription. These results are reviewed in relation to other known mechanisms of transcriptional regulation in eukaryotes.