The Esterase Isozymes of Tetrahymena: Their Distribution in Isolated Cellular Components and Their Behavior during the Growth Cycle.

In recent years molecular heterogeneity of proteins has often been observed, particularly for enzymes, where the existence of multiple molecular forms has been extensively domumented ( Wrbblewski, '61). Markert and Mdler ('59) introduced the concept of an isozyme, which they defined as one of the "molecular forms in which proteins may exist with the same enzymatic specificity" within a single organism. This term has gained wide popularity and has been used in an operational sense to cover any series of enzymes with roughly similar properties regardless of genetic relationship. In nearly all cases the genetic relationships are not known. However, where the genetic relationships are known, informative distinctions among degrees of relationship can be drawn (Allen, Misch, and Morrison, '63a, '63b). In this paper, I shall use the term isozyme to apply to one of a series of molecular forms of an enzyme controlled by a single gene. Two examples of this type of isozyme are the alkaline phosphatases of Escherichia coli (Bach et al., '61; Levinthal, Signer, and Fetherolf, '62) and certain esterases of variety 1 of Tetrahymena pyrifornis ( S . L. Allen, '60, '61a). In T. pyrifornis the esterase isozymes are resolved by electrophoresis in starch gels (S. L. Allen, '60). There are two groups of isozymes that are discontinuously distributed among the various inbred strains. Each group is controlled by an allele at the E-1 locus (S. L. Allen, '61a) and the E-1 locus is linked to the mt (mating type) locus (S . L. Allen, '64). Both groups of isozymes are present in heterozy gotes. Clonal analysis indicated that all the isozymes of a group are present in single