Structure and expression of a gene encoding the large subunit of ribulose-1,5-bisphosphate carboxylase (rbcL) in the colourless euglenoid flagellate Astasia longa

Abstract

A gene encoding the large subunit of ribulose-1,5-bisphosphate carboxylase (Rubisco) was identified on a circular 73 kb DNA from the colourless euglenoid flagellate Astasia longa. The rbcL gene of Astasia extends over 3968 bp. It is a split gene interrupted by seven introns as compared to nine intervening sequences in the rbcL gene of the phylogenetically related Euglena gracilis. Coding sequences as well as the positions of the introns within this gene are highly conserved in comparison with the Euglena rbcL except that two introns are missing in Astasia. The alignment of the amino acid sequences deduced from the nucleotide sequences of rbcL of Astasia and Euglena shows 82% identical amino acids whereas 15% of the amino acids represent conservative changes. A 1.5 kb transcript of the rbcL gene was revealed by northern blot analysis of Astasia RNA. By immunoblot analysis the gene product of rbcL was detected as a 53 kDa polypeptide. Genes for components of the chloroplast transcriptional and translational systems encoded by chloroplast DNA of plants and green algae are conserved on the 73 kb DNA of Astasia [24, 25, 26]. From our finding that Astasia obviously is capable of synthesizing the Rubisco large subunit one must conclude that these genes are expressed and form functional plastid transcriptional and translational systems.