ISOLATION, CHARACTERIZATION, AND CHROMOSOME MAPPING OF AN ACTIN GENE FROM THE PRIMITIVE GREEN ALGA, NANNOCHLORIS BACILLARIS (CHLOROPHYCEAE)

Abstract

Historically, the genus Nannochloris has been classified using the morphology of cell division, although the mechanics of division remain relatively poorly understood. Nannochloris bacillaris reproduces by binary fission. Microscopic observation with fluorescein isothiocyanate‐phalloidin showed that actin filaments localized near the nucleus and appeared as a ring‐ or beltlike structure in the septum‐forming area in the middle of the cell during cell division. In primitive unicellular Chlorophyta such as N. bacillaris, actin is also thought to play important roles in nuclear migration and cell division. The N. bacillaris actin gene has three exons and two introns defined by two exon–intron junctions with splice site consensus sequences. The two introns are located at codons specifying amino acids 3/4 and 47/48. One of these, intron position 3/4, is conserved in the actin gene of Saccharomyces cerevisiae. The actin gene product was predicted to be 378 amino acids long with an estimated molecular weight of 42 kDa. There is only one copy of the actin gene in the N. bacillaris genome. Nannochloris bacillaris has 14 chromosomes that range in size from 230 kb to 3000 kb, and the total size of the genome was estimated to be 20.3 Mb. The actin gene is on either chromosome XI or XII. In a phylogenetic tree based on the actin gene sequence, N. bacillaris diverged before the divergence of Volvox, Chlamydomonas, and higher plants, and very shortly after the radiation of the Rhodophyta.