Ribulose-1,5-bisphosphate carboxylase, a marker for chloroplast species specificity in Acetabularia

Interspecific and intergeneric nucleo-cytoplasmic hybrids may be readily produced under controlled conditions from Acetabularia and other related Dasycladaceae. The role of the nuclear genome has been well documented, whereas the lack of a suitable chloroplast marker has hindered the analysis of the possible interaction of the two plastid genomes in these hybrids. To this end, the species specificity of the chloroplast genome-encoded large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) was studied by two-dimensional electrophoresis in nine species of Dasycladaceae. The large subunit has a molecular weight (±S.D.) of 52000±2000 and multiple isoelectric variants with isoelectric points between pH 5.3 and 5.7. The nine species could be divided into two distinct groups on the basis of the isoelectric focusing patterns. In both groups, individual isoelectric variants were found to give rise to the other isoelectric variants characteristic for that group upon re-electrofocusing. Mixing experiments confirmed the observed uniqueness of the two groups of species. These results indicate that the two-dimensional pattern of the large subunit of ribulose-1,5-bisphosphate carboxylase is a valid marker for chloroplast species specificity in Acetabularia