THE PSAA OPERON PRE-MESSENGER-RNA OF THE EUGLENA-GRACILIS CHLOROPLASTIS PROCESSED INTO PHOTOSYSTEM-I AND PHOTOSYSTEM-II MESSENGER-RNAS THAT ACCUMULATE DIFFERENTIALLY DEPENDING ON THE CONDITIONS OF CELL-GROWTH

The chloroplast genome of Euglena gracilis contains a psaA operon whic h encodes a lysine tRNA gene, trnK; psaA and psaB photosystem I genes, and psbE, psbF, psbL and psbJ photosystem II genes. The pre-mRNA of t he psaA operon undergoes a complex processing pathway of 5' and 3' tRN A processing, splicing of 11 group II introns and one group II twintro n, plus three intercistronic RNA cleavage events. The accumulated tran scripts of the psaA operon have been characterized by Northern hybridi zation, S1 nuclease analysis and primer extension RNA sequencing. The mature 5' end of the psaA-psaB-psbE-psbF-psbL-psbJ hexacistronic trans cript lies 8 nt downstream of the trnK gene, and is the result of inte rcistronic trnK-psaA cleavage. Other intercistronic processing events occur between the psaA and psaS genes and the psaS and psbE genes. Pro cessing at the latter site produces a dicistronic mRNA of PSI genes an d a tetracistronic mRNA of PSII genes. The PSI dicistronic transcript is further processed to monocistronic psaA and psaS mRNAs. Secondary s tructural motifs within the intercistronic regions may be recognition sites for processing. The steady-state levels of psaA operon mRNAs fro m Euglena grown under several different conditions have been determine d. Accumulated transcripts from all growth conditions are spliced, and a proportion are also processed at the intercistronic sites. The prod ucts of intercistronic processing increase from heterotrophic dark- to heterotrophic light-grown Euglena, and from heterotrophic light- to p hotoautotrophic light-grown Euglena. The differential accumulation of psaA operon mRNAs may be a means of chloroplast gene regulation or, al ternatively, a consequence of gene expression during chloroplast devel opment.