MESSENGER-RNA STABILITY IS REGULATED BY A CODING-REGION ELEMENT AND THE UNIQUE 5' UNTRANSLATED LEADER SEQUENCES OF THE 3 SYNECHOCOCCUS-PSBATRANSCRIPTS

The psbAl and psbAlll transcripts in Synechococcus sp. strain PCC 7942 are subject to accelerated turnover when cells are exposed to high li ght intensities, but psbAll message stability is unaffected. We used a psbAl 'minigene' which has a part of the coding sequence removed as a reporter gene in order to identify the cis-acting elements of the tra nscript that determine stability. While engineering the minigene to op timally mimic the native gene, we identified a stabilizer element with in the open reading frame, corresponding to the coding region for the first membrane span of the D1 protein, the presence of and translation through which was essential for normal psbA mRNA stability. We propos e that this stabilizer is a site for ribosome pausing, and that accumu lation of ribosomes on the transcript upstream of the pause site incre ases stability. To identify the elements that regulate the differentia l responses of the psbA transcripts to high-light growth, sequences fr om psbAll and psbAlll were substituted in the psbAl minigene reporter. The chimeric reporter transcripts established that the psbAl and psbA lll untranslated leaders determine the faster turnover of these messag es. The untranslated leader regions of the psbA transcripts may regula te mRNA stability by modulating translation and thereby stability, or by recruiting RNA-binding proteins that affect mRNA turnover more dire ctly.