Characterization of a phototolerant mutant of Synechocystis sp. PCC 6803 created by random mutagenesis of psbAII gene.

Photosensitivity and photosynthetic characteristics have been analyzed in wild type (KC) and its psbAII mutant (I6) of Synechocystis having three point amino acid substitutions, i.e., N322I, I326F and F328S, which are localized in the C-terminal extension of D1 protein of the photosystem II reaction center. Wild type and mutant cells show almost an identical growth pattern under normal/low light (30 mumol m-2s-1, 30 degrees C) liquid culture (BG-11) condition. However, upon shifting the cultures to high light (500 mumol m-2s-1, 30 degrees C), these two types of cells exhibit entirely different growth characteristics, i.e., the mutant cells continue to grow normally whereas, the control cells fail to adapt the light stress and eventually resulting in complete loss of the photosynthetic pigments. On the other hand, a quick loss in the Fv/Fm value with half--decay time of about 30 min is observed in the mutant, in contrast to 120-130 min in case of control, upon shifting to high light conditions. In spite of this, mutant cells are able to adapt and grow well under prolonged high light exposure even after losing a major part of the variable yield of chlorophyll fluorescence (Fv/Fm). The high light treatment also induced decrease in the level of D1 protein in the mutant. However, half-decay time for D1 is much longer (approximately 10 hr) than that of variable fluorescence. Thus, the mutant cells have shown an unique way for cell growth and maintenance under high light even after losing Fv/Fm and photosynthetic oxygen evolving capacity as well as D1 content to a great extent. Therefore, these results could extend an interesting insight to understand the coordination of physiological, biochemical and molecular mechanisms regulating phototolerance of the photosynthetic organisms.

Biogenesis and assembly of photosystem I.

Photosystem I (PS I) is a multisubunit membrane protein complex consisting of 11 to 14 different subunits. In addition, several cofactors, such as chlorophylls, phylloquinones, carotenoids and iron-sulfur clusters are bound by this complex. We now have a detailed understanding of the structural basics, yet we know very little about the molecular details of the assembly process that finally yields functional PS I. Moreover, not much is known about the molecular dynamics of PS I in the thylakoid membrane or its regulated degradation. These areas have become the focus of recent work and first results have emerged. In this minireview we describe the latest findings in this fascinating and rapidly evolving field.

DNA polymorphism in Cab locus of tomato induced by tissue culture.

Plants were regenerated from callus induced from leaf disc explants of a tomato F1 hybrid heterozygous for three marker loci anthocyaninless (a), without anthocyanin (aw), and hairless (hl). Regenerants were studied for somaclonal variation at the phenotypic level by scoring for variation in the marker loci, and at the DNA level by probing geomic DNA blots with a chlorophyll a/b binding protein (Cab-3C) cDNA sequence. While no variation was observed at the phenotypic level in over 950 somaclones studied, DNA polymorphism for the Cab locus could be detected in two out of 17 somaclones tested. Tissue culture induced variation at the phenotypic level for specific loci is very low (less than 0.001 for a, aw or hl) but DNA sequence changes are induced at much greater frequency (approximately 0.1 for a multicopy gene family such as Cab).