DEVELOPMENTAL AND CIRCADIAN CONTROL OF THE CAPACITY FOR DELTA-AMINOLEVULINIC-ACID SYNTHESIS IN GREEN BARLEY

The synthesis of delta-aminolevulinic acid (delta-ALA) is a key step i n the regulation of tetrapyrrole synthesis. To study the developmental ly and circadian-clock controlled mechanism that co-ordinates synthesi s of chlorophylls and chlorophyll-binding proteins, delta-ALA-synthesi sing capacity was analysed in barley (Hordeum vulgare L,) primary leav es grown under dark: light or constant light conditions. The delta-ALA -forming activity oscillated within 24 h with a maximum at the transit ion of dark to light and a minimum 12 h later, indicating the involvem ent of the circadian oscillator during development. The capacity for d elta-ALA synthesis increased transiently in the middle of barley prima ry leaves. The delta-ALA-forming-activity correlated well with the pre viously published steady-state level of mRNA for light-harvesting chlo rophyll-binding proteins in space and time; this supports the view of a co-ordinate synthesis of chlorophyll and pigment-binding proteins. S teady-state levels of mRNAs encoding the three enzymes of the delta-AL A-synthesising pathway and of proteins for glutamyl-tRNA reductase (Gl uTR) and glutamate 1-semialdehyde aminotransferase (GSA AT; EC 5.4.3.8 ) were analysed for their developmental and circadian expression in ba rley leaves. The contents of GluTR mRNA and protein cycled parallel to the changes in delta-ALA-forming activity. The levels of GSA AT mRNA oscillated in an opposite phase, but the protein content did not show substantial oscillation under diurnal and circadian growth conditions. No circadian oscillation was detected for glutamyl tRNA synthase (Glu RS; EC 6.1.1.17). Maximal GluTR mRNA content and protein was observed in the middle (segments 3 and 4) of the barley primary leaves. The dev elopmentally controlled expression of GluTR therefore differs from tha t of GSA AT and GluRS, but resembles the capacity for delta-ALA synthe sis in a barley leaf gradient. These data indicate that the oscillatin g, light-dependent and spatial expression of GluTR mRNA might contribu te to the regulated formation of the chlorophyll precursor delta-ALA.