We have cloned the Arabidopsis thaliana uridine 5＇-monophosphate synthase (UMP synthase) locus and characterized transcript levels of this gene as well as the transcript encoding a novel F-box protein found upstream of UMP synthase, Skp1-interacting partner 5 (SKIP5). The 3＇end of the Skips gene is 405 bp from the 5＇end of the UMP synthase gene. To determine whether these proximate genes are coordinately or independently regulated, we have used an RT-PCR method to quantitate the transcript levels of each gene relative to 18S ribosomal RNA. Previous work has demonstrated an up-regulation of the UMP synthase gene in tobacco callus in the presence of compounds such as 5-fluoro-orotate and aminopterin that induce thymine starvation. Here, we present results showing that both the UMP synthase and SKIP5 genes are coordinately up-regulated in the presence of fluoroorotic arid.