Transposition of a Ty3 GAG3-POL3 fusion mutant is limited by availability of capsid protein.

Ty3 encodes structural proteins in its upstream open reading frame (GAG3) and catalytic proteins in an overlapping open reading frame (POL3). As is the case for retroviruses, high levels of structural protein versus catalytic proteins are synthesized and we show here that catalytic proteins are derived from a GAG3-POL3 fusion polyprotein. To evaluate the relative contributions of structural and catalytic components of the Ty3 particle, we perturbed the balance of these proteins by fusing the GAG3 and POL3 frames. This fusion Ty3 was capable of complementing low levels of transposition of a donor Ty3 which contained only cis-acting sequences required for transposition. Examination of extracts of cells expressing the GAG3-POL3 fusion mutant showed that particle formation differed qualitatively and quantitatively from viruslike particle formation by wild-type Ty3. Suprisingly, expression of 238 codons of GAG3, encoding only capsid protein, complemented transposition and particle formation defects of the fusion mutant, showing that the limiting deficiency was in capsid, and not in nucleocapsid, function. In addition, protein containing the capsid domain expressed alone accumulated in the same particulate fraction as viruslike particles, showing that it was sufficient for particle formation. The activity of the Ty3 fusion mutant contrasts with the inviability of mutant retroviruses in which gag and pol frames were fused and argues that retrotransposons tolerate considerable variation in the nucleoprotein complexes that permit replication and integration.

MAL6 of Saccharomyces: a complex genetic locus containing three genes required for maltose fermentation.

The MAL6 locus is one of five closely related unlinked loci, any one of which is sufficient for fermentation of maltose in Saccharomyces. Previous genetic analysis indicated that this locus is defined by two complementation groups, MALp and MALg. MALp reportedly is a regulatory gene required for inducible synthesis of the two enzymatic functions needed for fermentation: maltose permease and maltase. We have investigated the physical and genetic structure of the MAL6 locus, which has been isolated on a recombinant DNA plasmid. One subclone of the region, pDF-1, was found to encode a single transcribed region and to contain the MALp gene. A second subclone, p1, was shown to contain the MALg function but surprisingly had not one but two maltose-inducible transcripts. Subclones having only one of these transcribed regions lacked MALg activity. The three transcribed regions have been named MAL61 and MAL62, which correspond to MALg, and MAL63, which corresponds to MALp. This clustered arrangement of a regulatory gene adjacent to the sequences it controls has not previously been described in eukaryotes and is reminiscent of bacterial operons except that the messenger RNA molecules are not polycistronic.