Molecular cloning and expression analysis of the mitochondrial pyruvate dehydrogenase from maize

Four cDNAs, one encoding an cu-subunit and three encoding beta-subunits of the mitochondrial pyruvate dehydrogenase, were isolated from maize (Zea may s L.) libraries. The deduced amino acid sequences of both alpha- and beta-s ubunits are approximately 80% identical with Arabidopsis and pea (Pisum sat ivum L.) homologs. The mature N terminus was determined for the beta-subuni t by microsequencing the protein purified from etiolated maize shoot mitoch ondria and was resolved by two-dimensional gel electrophoresis. This ingle isoelectric species comprised multiple isoforms. Both alpha- and beta-subun its are encoded by multigene families in maize, as determined by Southern-b lot analyses. RNA transcripts for both alpha- and beta-subunits were more a bundant in roots than in young leaves or etiolated shoots. Pyruvate dehydro genase activity was also higher in roots (5-fold) compared with etiolated s hoots and leaves. Both subunits were present at similar levels in all tissu es examined, indicating coordinated gene regulation. The protein levels wer e highest in heterotrophic organs and in pollen, which contained about 2-fo ld more protein than any other organ examined. The relative abundance of th ese proteins in nonphotosynthetic tissues may reflect a high cellular conte nt of mitochondria, a high level of respiratory activity, or an extra plast idial requirement for acetate.