A SINGLE-GENE ENCODES 2 DIFFERENT TRANSCRIPTS FOR THE ADP-GLUCOSE PYROPHOSPHORYLASE SMALL-SUBUNIT FROM BARLEY (HORDEUM-VULGARE)

ADP-glucose pyrophosphorylase (AGPase), a heterotetrameric enzyme comp osed of two small and two large subunits, catalyses the first committe d step of starch synthesis in plant tissues. In an attempt to learn mo re about the organization and expression of the small-subunit gene of AGPase, we have studied the small-subunit transcripts as well as the s tructure of the gene encoding these transcripts in barley (Hordeum vul gare L. cv. Bomi). Two different transcripts (bepsF1 and blps14) were identified: bepsF1 was abundantly expressed in the starchy endosperm b ut not in leaves, whereas blps14 was isolated from leaves but was also found to be present at a moderate level in the starchy endosperm. The sequences for the two transcripts are identical over approx. 90% of t he length, with differences being confined solely to their 5' ends. In blps14, the unique 5' end is 259 nt long and encodes a putative plast id transit peptide sequence. For the 178-nt 5' end of bepsF1, on the o ther hand, no transit peptide sequence could be recognized. A lambda c lone that hybridized to the AGPase transcripts was isolated from a bar ley genomic library and characterized. The restriction map has suggest ed a complex organization of the gene, with alternative exons encoding the different 5' ends of the two transcripts followed by nine exons c oding for the common part of the transcripts. The sequence of a portio n of the genomic clone, covering the alternative 5'-end exons as well as upstream regions, has verified that both transcripts are encoded by the gene. The results suggest that the small-subunit gene of barley A GPase transcribes two different mRNAs by a mechanism classified as alt ernative splicing.