Removal of a 712 base pair (bp) sequence following the coding region of a human micro opioid receptor (hmuOR) cDNA unexpectedly increased expression of the receptor protein. A series of 3'-noncoding region deletion mutants revealed that at least three discrete regions following the stop codon influenced receptor expression levels. Deletion of the 205-bp 5'-noncoding region immediately preceding the Kozak sequence doubled receptor expression relative to wild type, and simultaneous deletion of 5'- and 3'-noncoding regions increased expression several fold. The hmuOR noncoding regions may participate in a regulatory mechanism that controls the number of cell surface receptors.
