Alterations in Barley Ribulose-1,5-bisphosphate Carboxylase/ Oxygenase Activase Gene Expression during Development and in Response to Illumination

Two genes encode Rbu-P₂-carboxylase activase in barley (RcaA and RcaB): RcaA encodes polypeptides of 46 and 42 kDa, which are generated by the alternatively spliced RcaA1 and AcaA2 mRNAs, respectively; RcaB encodes a 42-kDa polypeptide (Rundle, S. J., and Zielinski, R. E. (1991) J. Biol. Chem. 266, 4677-4685). In the cellular differentiation gradient of the first leaf of barley, the three Rca mRNAs accumulate differentially. RcaA1 and A2 mRNAs accumulate predominantly in the mature, most photosynthetically active regions of the leaf in a pattern that parallels accumulation of total Rbu-P₂-carboxylase activase protein. However, the kinetics of accumulation of RcaA1 and RcaA2 mRNA differ slightly, indicating that either changes in RcaA pre-mRNA splicing or mRNA turnover occur during development. RcaB mRNA, in contrast, accumulates in the youngest and oldest cell populations at the base and tip of the leaf, respectively. In the mid-region of the leaf, the difference in accumulation between RcaA and RcaB mRNAs is largely attributable to differences in the rates of transcription of the two Rca genes. In this region of the leaf, the three Rca mRNAs accumulate differentially throughout the course of the diurnal cycle. Steady state levels of the three Rca mRNA species increase in parallel in response to increasing irradiance; these changes were accompanied by increased Rbu-P₂-carboxylase activase protein accumulation.