Clones encoding two different forms of plastid pyruvate kinase (PKp; EC 2.7.1.40) have been isolated from both castor and tobacco seed cDNA libraries. One form, designated PKpA, from castor was described in a previous report, and the tobacco homologue of PKpA has now been isolated. In addition, a second cDNA, designated PKpG, has been identified and sequenced in both species. Western blot analysis, using antibodies raised against protein overexpressed from these clones, indicates that they encode the two predominant polypeptides of plastid pyruvate kinase from developing castor endosperm. In castor, both PKpA and PKpG are encoded by single genes. In the allotetraploid Nicotiana tabacum, there are two copies of each, one derived from each of the progenitors of this species. The expression of the genes for PKpA and PKpG was examined in various tissues from both castor and tobacco. In castor, both forms are expressed in developing and germinating endosperm and in the root but neither is expressed in the leaf. In tobacco, both forms are expressed in developing seeds but in mature tissues, PKpA is most abundant in roots and PKpG in leaves.

Various tissues from both germinating and developing castor seeds (Ricinus communis L.) have been analyzed for the level of expression of the genes for the a-and ,-subunits of pyrophosphatedependent phosphofructokinase (PFP). In tissues in which PFP is expressed, there is a single mRNA species of approximately 2 kilobases for each of the subunits. In germinating endosperm, the gene for the a-subunit is expressed at an earlier time after imbibition than that for the j-subunit, whereas in developing castor seed endosperm, both genes are highly and coordinately expressed. During seedling development, there is tissue-specific expression of the two genes. Tissues in which there is a high level of mRNA correspond with tissues in which both subunits of PFP can be detected. The differential expression of the two subunit genes in germinating endosperm does not result in the presence of the asubunit polypeptide in the absence of the ,-subunit polypeptide. Southern analysis of castor genomic DNA indicates the presence of a single gene for both the a-and a-subunits of PFP in contrast with potato, in which there are at least two genes for each subunit.