In this study, we sought to further understand previously reported instabilities of plasmids expressing the U. californica acyl-ACP thioesterase (Voelker and Davies, 1994) by varying the copy number of a codon-optimized version of the gene (BTE) on plasmids having different origins of replication, in an effort to balance the level of active protein with possible metabolic burden and product toxicity (Jones et al., 2000).