Hmg-coA reductase gene family in cotton (Gossypium hirsutum L.): Unique structural features and differential expression of hmg2 potentially associated with synthesis of specific isoprenoids in developing embryos

As a first step towards understanding the biosynthesis of isoprenoids that accumulate in specialized pigment glands of cotton at the molecular level, two full-length genes (hmg1 and hmg2) were characterized encoding hmg-coA r eductase (HMGR; EC 1.1.1.34), the enzyme that catalyzes the formation of a key isoprenoid precursor. Cotton hmgr genes exhibited features typical of o ther plant genes, however, hmg2 encodes the largest of all plant HMGR enzym es described to date. HMG2 contains several novel features that may represe nt functional specialization of this particular HMGR isoform, Such features include a unique 42 amino acid sequence located in the region separating t he N-terminal domain and C-terminal catalytic domain, as well as an N-termi nal hydrophobic domain that is not found in HMG1 or other HMGR enzymes. DNA blot analysis revealed that hmg1 and hmg2 belong to small subfamilies that probably include homeologous loci in allotetraploid cotton (Gossypium hirs utum L.), Ribonuclease protection assays revealed that hmg1 and hmg2 are di fferentially expressed in a developmentally- and spatially-modulated manner during morphogenesis of specialized terpenoid-containing pigment glands in embryos, Induced expression of hmg2 coincided with a possible commitment t o sesquiterpenoid biosynthesis in developing embryos, although other develo pmental processes also requiring HMGR cannot be excluded.