The influence of sulfur deficiency on biomass production was analyzed in the four Chlorellaceae species, Chlorella vulgaris, Chlorella sorokiniana, Chlorella lobophora, and Parachlorella kessleri. Culturing under sulfur-deficient conditions promoted transient accumulation of starch followed by a steady increase in lipid storage. Transmission electron microscopy indicated an increase and decrease in starch granules and subsequent enlargement of lipid droplets under sulfur-deficient conditions. Chlorellaceae spp. accumulated 1.5-2.7-fold higher amounts of starch and 1.5-2.4-fold higher amounts of lipid under sulfur-deficient conditions than under sulfur-sufficient conditions. More than 75% of the fatty acids that accumulated in Chlorellaceae spp. under the sulfur-sufficient condition were unsaturated and culturing under sulfur-deficient conditions increased the saturated fatty acid content from 24.3% to 59.7% only in P. kessleri. These results indicate that the sequential accumulation of starch and lipid is a response to the sulfur depletion that commonly occurs in Chlorellaceae spp.

SUMMARYSeptins are a group of GTP-binding proteins that are multi-functional, with a well-known role in cytokinesis in animals and fungi. Although the functions of septins have been thoroughly studied in opisthokonts (fungi and animals), the function and evolution of plant/algal septins are not as well characterized. Here we describe septin localization and expression in the green algae Nannochloris bacillaris and Marvania geminata. The present data suggest that septins localize at the division site when cytokinesis occurs. In addition, we show that septin homologs may be found only in green algae, but not in other major plant lineages, such as land plants, red algae and glaucophytes. We also found other septin homolog-possessing organisms among the diatoms, Rhizaria and cryptomonad/haptophyte lineages. Our study reveals the potential role of algal septins in cytokinesis and/or cell elongation, and confirms that septin genes appear to have been lost in the Plantae lineage, except in some green algae.

A newly isolated mutant, mus-23, of Neurospora crassa was found to be highly sensitive to a wide variety of mutagens, including UV light, methyl methanesulfonate, 4-nitroquinoline 1-oxide, N-methyl-N'-nitro-N-nitrosoguanidine and tert-butyl hydroperoxide. This mutant was originally isolated as a mutant that could not grow on medium containing histidine. Meiosis and sporulation were defective in homozygous crosses between mus-23 haploids. The mus-23 gene is located on the right arm of LGII, between fl and trp-3. Analyses of epistasis between mus-23 and other mutations that cause defects in DNA repair indicated that the mus-23 gene belongs to the same DNA repair group as mei-3, which is the Neurospora homolog of the Saccharomyces cerevisiae gene RAD51. The double mutant carrying mus-23 and uvs-3 mutations was lethal. The mus-23 gene was cloned by complementation of the MMS-sensitive phenotype of the mus-23 mutant. The gene contained an open reading frame of 1578 bp and did not contain any introns. The molecular weight of the predicted mus-23 gene product was 60.4 kDa. Computer analyses revealed that the MUS23 protein has significant homology to Mre11p, which is known to be involved in recombinational repair in S. cerevisiae. The level of mus-23 transcripts increased significantly within 60 min of treatment with UV or MMS and then gradually decreased. The role of MUS23 protein in recombinational repair is discussed.