PCAL, A HIGHLY UNUSUAL TY1 COPIA RETROTRANSPOSON FROM THE PATHOGENIC YEAST CANDIDA-ALBICANS/

Retrotransposons are mobile genetic elements. They can transpose via t he reverse transcription of mRNA into double-stranded DNA (dsDNA) foll owed by the insertion of this dsDNA into new sites within the host gen ome. The unintegrated, linear, dsDNA form of retrotransposons is usual ly very rare. We report here the isolation of a retrotransposon from C andida albicans which is unusual in this respect. This element, which we have named pCal, was first identified as a distinct band when uncut C. albicans DNA was examined on an agarose gel. Sequence analysis of the cloned element revealed that it is a retrotransposon belonging to the Ty1/copia group. It is estimated that pCal produces 50 to 100 free , linear, dsDNA copies of itself per cell. This is a much higher level of expression than even that of the system in which Ty1 is expressed behind the highly active GAL1 promoter on a high-copy-number plasmid ( about 10 copies per cell). Another unusual feature of pCal is that its Pol enzymes are likely to be expressed via the pseudoknot-assisted su ppression of an upstream, in-phase stop codon, as has been shown for M oloney murine leukemia virus.