ROLE OF SIGNAL PEPTIDES IN TARGETING OF PROTEINS IN CYANOBACTERIA

Proteins of cyanobacteria may be transported across one of two membran e systems: the typical eubacterial cell envelope (consisting of an inn er membrane, periplasmic space, and an outer membrane) and the photosy nthetic thylakoids. To investigate the role of signal peptides in targ eting in cyanobacteria, Synechococcus sp. strain PCC 7942 was transfor med with vectors carrying the chloramphenicol acetyltransferase report er gene fused to coding sequences for one of four different signal pep tides. These included signal peptides of two proteins of periplasmic s pace origin (one from Escherichia coli and the other from Synechococcu s sp. strain PCC 7942) and two other signal peptides of proteins locat ed in the thylakoid lumen (one from a cyanobacterium and the other fro m a higher plant). The location of the gene fusion products expressed in Synechococcus sp. strain PCC 7942 was determined by a chloramphenic ol acetyltransferase enzyme-linked immunosorbent assay of subcellular fractions. The distribution pattern for gene fusions with periplasmic signal peptides was different from that of gene fusions with thylakoid lumen signal peptides. Primary sequence analysis revealed conserved f eatures in the thylakoid lumen signal peptides that were absent from t he periplasmic signal peptides. These results suggest the importance o f the signal peptide in protein targeting in cyanobacteria and point t o the presence of signal peptide features conserved between chloroplas ts and cyanobacteria for targeting of proteins to the thylakoid lumen.