Acyl carrier protein (ACP) is an essential cofactor for plant fatty acid synthesis. Three isoforms occur in barley seedling leaves. The genes Acl1 and Acl3 coding for the predominant ACP I and the minor ACP III, respectively, have been cloned and characterized as has a full-length cDNA for ACP III. Both genes, extending over more than 2.5 kb, have a conserved mosaic structure of four exons and three introns which result in mRNAs of ca. 900 bases. Alignment of the DNA sequences demonstrates that homology is restricted to the two exons coding for the mature protein whereas the remaining segments of the genes including the transit peptide-coding domains lack homology. Southern blot analyses demonstrate that Acl1 and Acl3 represent single copy genes located on chromosomes 7 and 1, respectively. Primer extension analyses identified multiple transcription start sites in both genes. The promoter regions are remarkably different; that of Acl3 resembles those for mammalian housekeeping genes in having a high G + C content plus three copies of an RNA polymerase II recognition GC element and in lacking correctly positioned TATA boxes. These features are in accordance with the hypothesis that Acl1 is specifically expressed in leaf tissue whereas Acl3 is a constitutively expressed gene.

Three cDNA clones for acyl carrier protein (ACP) isolated from a Lgtl I cDNA library prepared from greening barley seedling leaves using a 66'mer oligonucleotide probe have been sequenced, oACP 2 (371 bases) and pACP 11 (768 bases) code for the 90 amino acid chloroplast located ACP I. pACP I (520 bases) codes for a hitherto unknown protein with 83 amino acids that has been designated ACP Ill. The primary structure of its 27 amino acid partial transit peptide bears no obvious similarities to the transit peptide of ACP I. Northern analysis of immature caryopses' poly A+ RNA using the 66'mer probe reveals the presence of an ACP mRNA of significantly smaller size than the ACP mRNAs detectable in the leaf.

ACP2 is a component of plastid-located plant fatty acid synthetase. It binds acyl groups covalently via the prosthetic group, 4-phosphopantetheine, during the biosynthesis of fatty acids. Three isoforms have been identified in barley leaves (1, 3). Protein sequence data have been obtained for ACP I and 11 (3), and in addition genomic clones encoding ACP I and III have been characterized (2). Herein are presented cDNA clones for ACP II and localization of its structural gene Ac12.