TABLE 1.

Relationship of phenotypes of a prion and mutations in the gene encoding the protein^a

Non-Mendelian element	Phenotype due to:		Relation of the two phenotypes	Does replacing the gene restore the phenotype?
	Presence of non-Mendelian element	Chromosomal mutant that loses the element
M dsRNA	Killer +	Killer −	Opposite	No
mitDNA	Glycerol +	Glycerol −	Opposite	No
Suppressive petite mitDNA	Glycerol −	Glycerol −	Same	No
Prion	Defective	Defective	Same	Yes
[URE3]	USA uptake +	USA uptake +	Same	Yes
[PSI]	Suppressor ⇑	Suppressor ⇑	Same	Yes

^aUsually the presence of a nucleic acid replicon produces the opposite phenotype to that produced by a mutation in a gene needed for its propagation. However, there is a circumstance in which the phenotypes for a nucleic acid replicon and mutation in one of its maintenance genes could be the same. Certain deletion mutants of mitochondrial DNA (mitDNA) (called suppressive petites) make their presence known by efficiently eliminating the normal mitochondrial DNA and thus producing the same defective phenotype as the absence of the mitochondria DNA. Such a mutant mitochondrial DNA depends for its replication on the same chromosomal genes as does the wild-type mitochondrial DNA. The difference is that these mutant mitochondrial DNAs are dominant to the wild type, so that they appear as the presence of a non-Mendelian genetic element. However, this type of mutant may be distinguished from a prion by the result of replacing the chromosomal maintenance gene. Deletion of the potential prion protein produces the defective phenotype, and replacement of the gene restores the normal phenotype. Deletion of the gene needed for propagation of the defective mitDNA gives the defective phenotype, but replacement of the gene does not repair the defect, since mitochondrial DNA is still missing.