FUNCTION OF THE CHLAMYDOMONAS-REINHARDTII PETD 5' UNTRANSLATED REGIONIN REGULATING THE ACCUMULATION OF SUBUNIT-IV OF THE CYTOCHROME B(6) FCOMPLEX/

Translational control is an important regulatory mechanism in chloropl asts, and is thought to be mediated by cis-acting elements in the 5' u ntranslated regions (UTRs) of mRNAs. Chloroplast transformation was us ed 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 nonphotosynthetic pheno type, or were unable to grow photosynthetically. Among those transform ants with a wild-type phenotype were ones containing mutations in a pu tative 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 si tuated 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 dif ferent strains. It was concluded that specific regions of the petD 5' UTR mediate translational activation, and that mRNA stability may be l inked to translatability.