Nucleus-encoded, plastid-targeted acetolactate synthase genes in two closely related chlorophytes, Chlamydomonas reihardtii and Volvox carteri: phylogenetic origins and recent insertion of introns.

Acetolactate synthase (ALS) catalyzes the first committed step in the synthesis of branched-chain amino acids. In green plants and fungi, ALS is encoded by a nuclear gene whose product is targeted to plastids (in plants) or to mitochondria (in fungi). In red algae, the gene is plastid-encoded. We have determined the complete sequence of nucleus-encoded ALS genes from the green algae Chlamydomonas reinhardtii and Volvox carteri. Phylogenetic analyses of the ALS gene family indicate that the ALS genes of green algae and plants are closely related, sharing a recent common ancestor. Furthermore, although these genes are clearly of eubacterial origin, a relationship to the ALS genes of red algae and cyanobacteria (endosymbiotic precursors of plastids) is only weakly indicated. The algal ALS genes are distinguished from their homologs in higher plants by the fact that they are interrupted by numerous spliceosomal introns; plant ALS genes completely lack introns. The restricted phylogenetic distribution of these introns suggests that they were inserted recently, after the divergence of these green algae from plants. Two introns in the Volvox ALS gene, not found in the Chlamydomonas gene, are positioned precisely at sites which resemble "proto-splice" sequences in the Chlamydomonas gene.