EXPRESSION OF PSBA GENES PRODUCES PROMINENT 5' PSBA MESSENGER-RNA FRAGMENTS IN SYNECHOCOCCUS SP. PCC-7942

Expression of the psbA genes, which in the cyanobacterium Synechococcu s sp. PCC 7942 encode two different forms of the reaction centre D1 pr otein of photosystem II (D1:1 and D1:2), was studied under different l ight and temperature conditions. In addition to the mature 1200 nt psb A messages, three shorter mRNA fragments of 220, 320 and 900 nt were a lso found. All three mRNA fragments could be recognized by using diffe rent gene probes from the coding region of the psbAI gene, whereas the corresponding psbAII/III gene probes recognized only the 220 nt mRNA fragment. The 5' 320 nt mRNA fragment from the psbAI gene probably rep resents a degradation product, since the corresponding 3' 900 nt psbAI mRNA fragment was also detected. By contrast, the 5' 220 nt mRNA frag ment of all psbA messages is suggested to be a truncated psbA transcri pt, since no corresponding 3' fragment was ever found. Inhibition of t ranslation either by a protein synthesis inhibitor or by a shift of ce lls to lower temperature, increased the number of 1200 nt psbAII/III m essages but the number of 5' 220 nt psbAII/III mRNA fragment increased even more dramatically. The first 66 bp after ATG, where the psbAI an d psbAII/III genes mostly differ from each other, also appeared import ant in determining the amount of produced truncated psbA transcripts, as evidenced by the expression of different tac-psbA constructs in the presence of protein syn thesis inhibitor. We suggest that both the ps bAI and the psbAII/III genes have a latent intragenic termination site and truncated psbA transcripts are produced at high levels under stre ss conditions when transcription becomes uncoupled from translation.Th is is to prevent wasting metabolic energy in the production of unused transcripts.