Cysteine biosynthesis in Chlamydomonas reinhardtii - Molecular cloning andregulation of O-acetylserine(thiol)lyase

A cDNA, Cys1ACr, encoding an isoform of O-acetylserine(thiol) lyase has bee n isolated from Chlamydomonas reinhardtii, using a PCR-based approach. The inclusion of dimethylsulfoxide in the PCR reaction has been demonstrated to be essential for the correct amplification of C. reinhardtii templates wit h complex secondary structures caused by a high G + C content. The deduced amino acid sequence exhibited highest similarity with plant O-acetylserine( thiol)lyase isoforms, indicating that the C. reinhardtii enzyme was structu rally more similar to higher plant O-acetylserine(thiol)lyase than to the c orresponding prokaryotic enzymes. The M-terminal extension present in Cys1A Cr showed several characteristics of an organellar transit peptide, with a length typical for C. reinhardtii. Southern blot analysis suggested that th e C. reinhardtii genome may contain a single copy of the organellar O-acety lserine(thiol)lyase gene. O-acetylserine(thiol)lyase activity was strongly induced by sulfur-deficient conditions (up to sevenfold the level observed in a sulfur-repleted cell culture) and required the presence of a nitrogen source. Northern blot analysis showed a different pattern of regulation of Cys1ACr to that observed at the activity level. To obtain an increase of tr anscript abundance a longer period of sulfur limitation was required, reach ing a maximum level of approximate to threefold Cys1ACr mRNA when compared with the level of a sulfate-grown culture.