Post-transcriptional RNA processing and translational regulations are important steps for gene expression. To analyze the 5′UTRof psbA that enhances translation of the sweet protein monellin in chloroplasts, we cloned the monellin gene, with and without thepsbA 5′UTR, into the chloroplast expression vector for chloroplast transformation. Transgenic plants were identified as being transplastomic via PCR and Southern blot analyses. We also observed non-specific recombination during tobacco chloroplast transformation. Analyses of the transcription patterns showed that intercistronic cleavage of the psbA mRNA 5′ untranslated (UTR) region was functional at the mature stage, with the monocistronic mRNA ofmonellin increasing while its dicistronic mRNA decreased. Moreover, monellin accumulation accounted for 2.3% of the total soluble protein at the mature stage, but only 1.3% at the young stage in transplastomic lines that contained the 5′UTRof psbA. These results suggest that activation of the endonucleolytic cleavage of thepsbA 5′UTR element depends on chloroplast developmental conditions, and that it enhances the accumulation of sweet protein monellin in those chloroplasts.