Two loci control phytoglycogen production in the monocellular green alga Chlamydomonas reinhardtii

The STA8 locus of Chlamydomonas reinhardtii was identified in a genetic scr een as a factor that controls starch biosynthesis. Mutations of STA8 cause a significant reduction in the amount of granular starch produced during nu trient limitation and accumulate phytoglycogen. The granules remaining in s ta8 mutants are misshapen, and the abundance of amylose and long chains in amylopectin is altered. Mutations of the STA7 locus, which completely lack isoamylase activity, also cause accumulation of phytoglycogen, although sta 8 and sta7 mutants differ in that there is a complete loss of granular star ch in the latter. This is the first instance in which mutations of two diff erent genetic elements in one plant species have been shown to cause phytog lycogen accumulation. An analytical procedure that allows assay of isoamyla se in total extracts was developed and used to show that sta8 mutations cau se a 65% reduction in the level of this activity. All other enzymes known t o be involved in starch biosynthesis were shown to be unaffected in sta8 mu tants. The same amount of total isoamylase activity (approximately) as that present in sta8 mutants was observed in heterozygous triploids containing two sta7 mutant alleles and one wild-type allele. This strain, however, acc umulates normal levels of starch granules and lacks phytoglycogen. The tota l level of isoamylase activity, therefore, is not the major determinant of whether granule production is reduced and phytoglycogen accumulates. Instea d, a qualitative property of the isoamylase that is affected by the sta8 mu tation is likely to be the critical factor in phytoglycogen production.