Nucleotide (nt) sequence of the large subunit (LS) gene of ribulose-1,5-bisphosphate carboxylase/oxygenase from the green alga, Chlamydomonas moewusii, and mapping of transcription ends was achieved by two new strategies. The deduced LS sequence of 475 amino acid residues was compared with similar genes from six other species; cyanobacteria, land plants and a related alga (C. reinhardtii). The most conserved regions are the three ribulose bisphosphate binding sites and the CO2 activator site. The nt sequence conservation outside the coding region is limited to only three segments within the 5'-flanking region: a region of tandem repeats, TATAA box and ribosome-binding site. Termination point of transcription is an 'A' residue 3' to the first of two 18-nt inverted repeats, which has the potential to form a stem-loop hairpin structure. The possible role of these potential regulatory features for transcription and translation, and similar structures in other LS genes is presented.