Changes in transfer ribonucleic acids accompanying encystment in Acanthamoeba castellanii

CCA-Addition Gone Wild: Unusual Occurrence and Phylogeny of Four Different tRNA Nucleotidyltransferases in Acanthamoeba castellanii

tRNAs are important players in the protein synthesis machinery, where they act as adapter molecules for translating the mRNA codons into the corresponding amino acid sequence. In a series of highly conserved maturation steps, the primary transcripts are converted into mature tRNAs. In the amoebozoan Acanthamoeba castellanii, a highly unusual evolution of some of these processing steps was identified that are based on unconventional RNA polymerase activities. In this context, we investigated the synthesis of the 3'-terminal CCA-end that is added posttranscriptionally by a specialized polymerase, the tRNA nucleotidyltransferase (CCA-adding enzyme). The majority of eukaryotic organisms carry only a single gene for a CCA-adding enzyme that acts on both the cytosolic and the mitochondrial tRNA pool. In a bioinformatic analysis of the genome of this organism, we identified a surprising multitude of genes for enzymes that contain the active site signature of eukaryotic/eubacterial tRNA nucleotidyltransferases. In vitro activity analyses of these enzymes revealed that two proteins represent bona fide CCA-adding enzymes, one of them carrying an N-terminal sequence corresponding to a putative mitochondrial target signal. The other enzymes have restricted activities and represent CC- and A-adding enzymes, respectively. The A-adding enzyme is of particular interest, as its sequence is closely related to corresponding enzymes from Proteobacteria, indicating a horizontal gene transfer. Interestingly, this unusual diversity of nucleotidyltransferase genes is not restricted to Acanthamoeba castellanii but is also present in other members of the Acanthamoeba genus, indicating an ancient evolutionary trait.

Fate of internalized Campylobacter jejuni and Mycobacterium avium from encysted and excysted Acanthamoeba polyphaga

Association of the water- and foodborne pathogen Campylobacter jejuni with free-living Acanthamoeba spp. trophozoites enhances C. jejuni survival and resistance to biocides and starvation. When facing less than optimal environmental conditions, however, the Acanthamoeba spp. host can temporarily transform from trophozoite to cyst and back to trophozoite, calling the survival of the internalized symbiont and resulting public health risk into question. Studies investigating internalized C. jejuni survival after A. castellanii trophozoite transformation have neither been able to detect its presence inside the Acanthamoeba cyst after encystation nor to confirm its presence upon excystation of trophozoites through culture-based techniques. The purpose of this study was to detect C. jejuni and Mycobacterium avium recovered from A. polyphaga trophozoites after co-culture and induction of trophozoite encystation using three different encystation methods (Neff's medium, McMillen's medium and refrigeration), as well as after cyst excystation. Internalized M. avium was used as a positive control, since studies have consistently detected the organism after co-culture and after host excystation. Concentrations of C. jejuni in A. polyphaga trophozoites were 4.5 × 10⁵ CFU/ml, but it was not detected by PCR or culture post-encystation. This supports the hypothesis that C. jejuni may be digested during encystation of the amoebae. M. avium was recovered at a mean concentration of 1.9 × 10⁴ from co-cultured trophozoites and 4.4 × 10¹ CFU/ml after excystation. The results also suggest that M. avium recovery post-excystation was statistically significantly different based on which encystation method was used, ranging from 1.3 × 10¹ for Neff's medium to 5.4 × 10¹ CFU/ml for refrigeration. No M. avium was recovered from A. polyphaga cysts when trophozoites were encysted by McMillen's medium. Since C. jejuni internalized in cysts would be more likely to survive harsh environmental conditions and disinfection, a better understanding of potential symbioses between free-living amoebae and campylobacters in drinking water distribution systems and food processing environments is needed to protect public health. Future co-culture experiments examining survival of internalized C. jejuni should carefully consider the encystation media used, and include molecular detection tools to falsify the hypothesis that C. jejuni may be present in a viable but not culturable state.

Acanthamoeba strains lose their abilities to encyst synchronously upon prolonged axenic culture

To evaluate the influence of prolonged axenic culture on the encystment capacity of Acanthamoeba spp., the encystment potential of four closely related Acanthamoeba strains, subcultured axenically for different periods of time, was evaluated comparing five encystment media. Media with more alkaline pH values were slightly more effective; however, the composition of the respective encystment medium had only limited influence on the encystment potential, while a strong correlation of losses in encystment potential and times strains had been cultured axenically was demonstrated. Furthermore, our results indicate that losses in encystment potential occur shortly after transfer into axenic culture to remain constant over many years.

Isolation and characterization of a pleiotropic glucose repression resistant mutant of Saccharomyces cerevisiae

A new mutation has been described which confers resistance to catabolite repression in Saccharomyces cerevisiae. The mutant allele, termed grr-1 for glucose repression-resistant, is characterized by insensitivity to glucose repression for the cytoplasmic enzymes invertase, maltase, and galactokinase, as well as the mitochondrial enzyme cytochrome c oxidase. Hexokinase levels in grr-1 mutants are approximately 3-fold higher than the corresponding activity of the parental strain. Although the grr-1 allele is expressed phenotypically similarly to the hex-1 (hxk-2) and hex-2 mutations described by Entian et al. (1977) and Zimmermann and Scheel (1977) respectively, we have shown genetically and physiologically that grr-1 represents a new class of mutation.

Changes in transfer ribonucleic acid population of Acanthamoeba castellanii during growth and encystment

Fifteen aminoacyl-transfer ribonucleic acids (tRNA's) from vegetative cells (trophozoites) and mature cysts of Acanthamoeba castellanii were compared by reversed-phase 5 chromatography. Little or no differences were detected in reversed-phase 5 chromatography elution profiles of alanyl-, arginyl-, isoleucyl-, phenylalanyl-, prolyl-, seryl-, threonyl-, tryptophanyl- and valyl-tRNA's. Significant differences in the relative proportions of isoaccepting species of leucyl-, lysyl-, methionyl-, aspartyl-, histidyl-, and tyrosyl-tRNA's were observed. Based upon the criterion of cyanogen bromide reactivity with the modified nucleoside queuosine, the content of queuosine in aspartyl-tRNA of A, castellanii is significantly greater in mature cysts than in trophozoites. The similarity of change in reversed-phase 5 chromatography elution profiles of aspartyl-, histidyl-, and tyrosyl-tRNA suggests that a common mechanism is responsible for alterations in the chromatographic patterns.