Function of the Chlamydomonas reinhardtii petd 5' untranslated region in regulating the accumulation of subunit IV of the cytochrome b6/f complex

Translational control is an important regulatory mechanism in chloroplasts, and is thought to be mediated by cis-acting elements in the 5' untranslated regions (UTRs) of mRNAs. Chloroplast transformation was used to replace the wild-type Chlamydomonas reinhardtii petD 5' UTR with mutated versions. Transformants containing altered 5' UTRs had either a wild-type photosynthetic phenotype, a leaky non-photosynthetic phenotype, or were unable to grow photosynthetically. Among those transformants with a wild-type phenotype were ones containing mutations in a putative Shine-Dalgarno sequence element. The results indicate that two regions of the 362 nucleotide (nt) 5' UTR may act as positive elements for translation, one located between nt 150 and 200, and the other situated approximately 40 nt upstream of the start codon, at nt 320. In every case where translation was compromised, petD mRNA accumulated to a lower level than in wild-type cells, ranging from 15% to 60% in different strains. It was concluded that specific regions of the petD 5' UTR mediate translational activation, and that mRNA stability may be linked to translatability.