High transcription is associated with genetic instability, notably increased 
spontaneous mutation rates, which is a phenomenon termed 
Transcription-Associated-Mutagenesis (TAM). In this study, we investigated TAM 
using the chromosomal CAN1 gene under the transcriptional control of two strong 
and inducible promoters (pGAL1 and pTET) in Saccharomyces cerevisiae. Both pTET- 
and pGAL1-driven high transcription at the CAN1 gene result in enhanced 
spontaneous mutation rates. Comparison of both promoters reveals differences in 
the type of mutagenesis, except for short (-2 and -3 nt) deletions, which depend 
only on the level of transcription. This mutation type, characteristic of TAM, 
is sequence dependent, occurring prefentially at di- and trinucleotides repeats, 
notably at two mutational hotspots encompassing the same 5'-ACATAT-3' sequence. 
To explore the mechanisms underlying the formation of short deletions in the 
course of TAM, we have determined Can(R) mutation spectra in yeast mutants 
affected in DNA metabolism. We identified topoisomerase 1-deficient strains 
(top1Δ) that specifically abolish the formation of short deletions under high 
transcription. The rate of the formation of (-2/-3nt) deletions is also reduced 
in the absence of RAD1 and MUS81 genes, involved in the repair of Top1p-DNA 
covalent complex. Furthermore ChIP analysis reveals an enrichment of trapped 
Top1p in the CAN1 ORF under high transcription. We propose a model, in which the 
repair of trapped Top1p-DNA complexes provokes the formation of short deletion 
in S. cerevisiae. This study reveals unavoidable conflicts between Top1p and the 
transcriptional machinery and their potential impact on genome stability.