Just a word of warning to everyone. I have recently been trying to map S.Cerevisiae sequencing reads to a reference genome. The pre built index supplied by bowtie is built on the CYGD assembly and not the commonly used SGD assembly (strain S288C). I'm unsure if CYGD use a different strain or an older assembly but I have noticed differences (mainly SNPs) in the sequence files.
Shaun Webb
galaxy-dev@lists.galaxyproject.org