Several proteins in the yeast Saccharomyces cerevisiae act as prions, forming infectious amyloids that are transmissible to other cells during division and outcrossed mating. The parallel in‐register β‐sheet structure of yeast prions such as [PSI+] and [RNQ+] provides a template for conversion of normally unstructured prion domains into a β‐sheet with folds at sites determined by the template on the ends of the filament. Prion propagation is highly sequence specific, and polymorphisms can disrupt templating and abrogate prion transmission. To investigate intra‐species barriers for prion transmission in yeast, we sequenced prion genes of 80 wild isolates from a diverse array of geographic locations and ecological niches. Molecular population genetics was used to quantify nucleotide diversity and examine the evolutionary basis for heterogeneity of prion alleles. An array of indels, SNPs and premature stop codons were detected in wild isolates. Many novel prion haplotypes were characterized, and several variants blocked prion propagation, resulting in barriers to transmission between strains. Almost half of the wild strains analyzed were heterozygous at one or more loci, suggesting that outcrossed mating occurs quite frequently in yeast. It is possible that the observed prion sequence heterogeneity is a consequence of natural selection against prion infections in wild yeast populations.

The Arabidopsis atmak3-1 mutant was identified on the basis of a decreased effective quantum yield of photosystem II. In atmak3-1, the synthesis of the plastome-encoded photosystem II core proteins D1 and CP47 is affected, resulting in a decrease in the abundance of thylakoid multiprotein complexes. DNA array-based mRNA analysis indicated that extraplastid functions also are altered. The mutation responsible was localized to AtMAK3, which encodes a homolog of the yeast protein Mak3p. In yeast, Mak3p, together with Mak10p and Mak31p, forms the N-terminal acetyltransferase complex C (NatC). The cytoplasmic AtMAK3 protein can functionally replace Mak3p, Mak10p, and Mak31p in acetylating N termini of endogenous proteins and the L-A virus Gag protein. This result, together with the finding that knockout of the Arabidopsis MAK10 homolog does not result in obvious physiological effects, indicates that AtMAK3 function does not require NatC complex formation, as it does in yeast. We suggest that N-acetylation of certain chloroplast precursor protein(s) is necessary for the efficient accumulation of the mature protein(s) in chloroplasts.

The Arabidopsis atmak3-1 mutant was identified on the basis of a decreased effective quantum yield of photosystem II. In atmak3-1, the synthesis of the plastome-encoded photosystem II core proteins D1 and CP47 is affected, resulting in a decrease in the abundance of thylakoid multiprotein complexes. DNA array–based mRNA analysis indicated that extraplastid functions also are altered. The mutation responsible was localized to AtMAK3, which encodes a homolog of the yeast protein Mak3p. In yeast, Mak3p, together with Mak10p and Mak31p, forms the N-terminal acetyltransferase complex C (NatC). The cytoplasmic AtMAK3 protein can functionally replace Mak3p, Mak10p, and Mak31p in acetylating N termini of endogenous proteins and the L-A virus Gag protein. This result, together with the finding that knockout of the Arabidopsis MAK10 homolog does not result in obvious physiological effects, indicates that AtMAK3 function does not require NatC complex formation, as it does in yeast. We suggest that N-acetylation of certain chloroplast precursor protein(s) is necessary for the efficient accumulation of the mature protein(s) in chloroplasts.