Cryptic Elements

Despite their frequent occurrence, a killer phenotype could not be attributed to most nonautonomous elements (Table 2). Since, to date, no other alternative traits (other than killer toxin production and immunity) could be disclosed, they were considered cryptic elements (Cong et al. 1994; Fukuhara 1995). For some of them, entire or partial nucleotide sequence data are available and potential coding capacities were addressed by heterologous hybridization to pGKLl derived probes (Cong et al. 1994; Fukuda et al. 1997, 2004; Klassen et al. 2002). Accordingly, cryptic elements may either encode their own DNA polymerase (pDHL1, pDH1A) or are devoid of it (pPE1A); the latter holds true also for the nonautonomous killer elements (pPac1-2, pWR1A). Surprisingly, each of the nonautonomous plasmids contains an ORF encoding a polypeptide resembling the zymocin a|3-precursor, i.e., the conserved toxin carrier complex (see above). A detailed inspection, however, is only possible for pPE1A, since it is the only cryptic linear plasmid entirely sequenced. There is a mutation in the chitinase active site and no hydropho-bic region similar to zymocin p (seen in all linear plasmid encoded killer toxins) could be identified. Moreover, an additional secreted protein (like zymocin y) is not encoded. Thus, pPE1A unlikely encodes a killer toxin, which may have escaped detection. Nevertheless, the zymocin a-like protein encoded by pPE1A ORF2 is secreted and binds to chitin in vitro (Klassen et al. 2002).

In summary, there is convincing evidence for conserved zymocin a-like protein even in cryptic elements, though its function outside of a killer toxin complex remains obscure at present. Stable inheritance of such plasmids in many different yeast species could, however, hardly be understood in cases where there is no autoselection or an alternative positive effect for the host cell.

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