Identification of the catalytic subunit of cAMP-dependent protein kinase from the photosynthetic flagellate, Euglena gracilis Z11The nucleotide sequence data reported in this paper will appear in the DDBJ/EMBL/GenBank nucleotide sequence databases with the accession number AB021126.

AbstractA gene named epk2 that encodes the amino acid sequence of a protein kinase was identified from the photosynthetic flagellate, Euglena gracilis Z. Homology search and phylogenetic analysis revealed that the deduced amino acid sequence of epk2 is most similar to that of the catalytic subunit of cAMP-dependent protein kinase (PKA). Northern blot analysis showed that Euglena cells express a 1.4-kb transcript of this gene. When the EPK2 protein was coexpressed with the rat regulatory subunit of PKA in cultured mammalian cells, these two proteins were coimmunoprecipitated. The association of EPK2 and the rat regulatory subunit of PKA was not detected in the cell lysate incubated with cAMP. EPK2 immunoprecipitated from the transfected cells phosphorylated Kemptide, a synthetic peptide substrate for PKA, and the phosphorylation was inhibited by PKI, a PKA-selective protein kinase inhibitor. These results indicate that EPK2 is a PKA homologue in the photosynthetic flagellate, and this is the first evidence for the occurrence of the PKA catalytic subunit in photosynthetic organisms

Changes in Cell Wall-Bound Phenolic Acids in the Internodes of Submerged Floating Rice

Submergence induces rapid elongation of internodes in floating rice (Oryza sativa L.). We examined the distributions of p-coumaric, ferulic and 5-5-coupled diferulic acids ester-linked to cell walls along the axis of highest internodes of submerged and air-grown floating rice stem segments. The amounts of ferulic and 5-5-diferulic acids per cell wall weight were lowest around the intercalary meristem, and increased as the distance from the meristematic zone increased toward the upper part of the internode in both air-grown and submerged stem segments. The ratio of 5-5-diferulic acid to ferulic acid also increased toward the upper, old parts of internodes in both air-grown and submerged stem segments. These observations suggest that the feruloylation of cell wall polysaccharides and the formation of diferulic acids contribute to the cessation of internodal cell elongation and that the formation of diferulic acids in cell walls is controlled by the coupling reaction in addition to the feruloylation. The amounts of p-coumaric acid per unit length and per cell-wall weight were markedly low in the newly elongated region of submerged internodes, and closely correlated with cell-wall dry mass in both air-grown and submerged internodes, suggesting that the deposition of p-coumaric acid in cell walls is related to the formation of secondary cell walls in floating rice internodes

Relationship between the Deposition of Phenolic Acids in the Cell Walls and the Cessation of Rapid Growth in Internodes of Floating Rice

We examined the involvement of p-coumaric, ferulic and 5-5-coupled diferulic acids ester-linked to cell walls in determining the elongation rate of internodes of floating rice (Oryza sativa L.). When floating rice stem segments were exposed to air after 2 days of submergence, the elongation rate of internodes was reduced and the degree of reduction was greater in the light than in the dark, while the internodes of stem segments submerged further for a comparable period continued rapid elongation. The amounts of ferulic and 5-5-coupled diferulic acids in the cell walls in the elongation zone of internodes significantly increased during the first day after exposure to air either in light or darkness. The increase of these phenolics in the cell walls after exposure to air was also observed on the second day in light, but not in darkness. On the other hand, the amount of p-coumaric acid increased only slightly on the first day after exposure to air, but rapidly on the second day in light. This pattern of change in the amounts of p-coumaric acid resembled that in the cell-wall mass (dry weight). The application of sucrose to the segments in darkness increased the amounts of phenolics in the cell walls of internodes to almost the same amount as those in light. These results indicate that the accumulation of ferulic and 5-5-coupled diferulic acids in cell walls may be related to the cessation of internodal elongation in floating rice and that the synthesis of phenolics in the cell wall is caused partially by the provision of sugar in light