Regulation of Aspartate Kinase Isoenzymes in Barley Mutants Resistant to Lysine plus Threonine

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Abstract
Two homozygous mutant lines of barley R3202 (Lt1b/Lt1b) and R3004 (Lt2/Lt2), are resistant to lysine plus threonine. They contain aspartate kinase [AK] isoenzymes with lost or decreased feedback sensitivity to lysine in either isoenzyme AKII (R3202) or isoenzyme AKIII (R3004). A homozygous double mutant line (Lt1b/Lt1b, Lt2/Lt2) was constructed that grows vigorously on 8 mM lysine, 8 mM threonine and 1 mM arginine. Both AKII and AKIII from the double mutant have altered lysine sensitivities, identical to those previously observed in R3202 and R3004, respectively. Aspartate kinase activity in extracts of leaves, roots and the maturing endosperm of the double mutant was much less sensitive to lysine inhibition than the enzyme in comparable extracts of the parent cv. Bomi, suggesting that aspartate kinase is expressed in a similar manner in different tissues of barley. A further mutant, R2501, resistant to lysine plus threonine gave rise to a homozygous line (Lt1a/Lt1a), which was not previously possible. AKII isolated from the homozygous line was completely insensitive to 10 mM lysine; the combined action of 10 mM lysine and 0.8 mM S-adenosylmethionine inhibited it by 60%, demonstrating the retention of some of the regulatory characteristics of the wild type enzyme.