The putative Kluyveromyces lactis
zymocin target complex, TOT, from Saccharomyces cerevisiae comprises five Tot
proteins, four of which are
RNA polymerase II (RNAP II) Elongator subunits. Recently, two more Elongator subunit genes, ELP6 (TOT6) and ELP4 (TOT7), have been identified. Deletions of both TOT6 and TOT7 result in the complex tot phenotype, including resistance to
zymocin, thermosensitivity, slow growth and
hypersensitivity towards drugs, thus reinforcing the notion that TOT/Elongator may be crucial in signalling zymocicity. Mutagenesis of ELP3/TOT3, the Elongator
histone acetyltransferase (HAT) gene, revealed that
zymocin sensitivity could be uncoupled from Elongator wild-type function, indicating that TOT interacts genetically with
zymocin. To test the possibility that
zymocin functions by affecting RNAP II activity in a TOT/Elongator-dependent manner, global
poly(A)+ mRNA levels were found to decline drastically on
zymocin treatment. Moreover, cells overexpressing Fcp1p, the RNAP II
carboxy-terminal domain phosphatase, acquired partial
zymocin resistance, whereas cells underproducing RNAP II became
zymocin hypersensitive. This suggests that
zymocin may convert TOT/Elongator into a cellular
poison toxic for RNAP II function and eventually leading to the observed G1 cell cycle arrest.