Molecular biology and evolution of the Rubisco small subunit multi-gene family in Mesembryanthemum crystallinum.

Abstract

The genes that comprise the multi-gene family encoding the small subunit of the CO₂ fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the facultative Crassulacean Acid Metabolism (CAM) plant Mesembryanthemum crystallinum were isolated and nucleotide sequences were determined. The gene family is comprised of six genes of which four are expressed (rbcS-1, rbcS-2, rbcS-3 and rbcS-4) based on the isolation of corresponding cDNAs and northern analysis. Sequence analysis of the genes reveals influences of gene duplication, gene conversion and selection pressure in the evolution of the gene family. Five of the genes are located in a tandem array within 20 kbp. Two are identical throughout the coding regions and intron sequences. The promoter regions of five of the genes lack regulatory sequence elements conserved in other dicot species. Gene specific probes were used to determine the expression properties of the rbcS-1, rbcS-2, rbcS-3 and rbcS-4 genes during environmental stress, development and the diurnal cycle. The four genes are differentially expressed and are co-regulated in response to stress and during development. Steady state rbcS mRNA amounts are down regulated by NaCl and cold stress. Comparison of changes in transcriptional activities relative to steady state mRNA amounts revealed that NaCl causes increased rbcS RNA turnover. RbcS expression is transcriptionally down regulated coincident with the developmental enablement of CAM induction by stress. The regulation of rbcS expression by changes in RNA turnover is itself developmentally regulated. The rbcS genes of M. crystallinum are transcriptionally regulated during development and are regulated at the level of RNA turnover by stress.