A full-length cDNA (designated rcaII) encoding the Rubisco activase (RCA) of rice (Oryza sativa L.) has been cloned from a cDNA library constructed with mRNA from green leaves. Sequence analysis resulted in a reading frame of 432 amino acids with a calculated molecular mass of 47.9 kDa and an estimated isoelectric point of 5.97. The deduced amino acid sequence showed 74-89% identity with other Rubisco activases from higher plants. Two highly conserved motifs were identified. Southern blot analysis suggested the presence of a single rca gene in the rice genome. The accumulation of leaf rca mRNA was found to be regulated by an oscillating circadian rhythm, in rice plants grown in a light-dark photoperiod. To purify the rice RCA protein, total soluble protein from rice green leaves was fractionated by ammonium sulfate precipitation, followed by preparative gel electrophoresis. Two polypeptides, designated RCAI and RCAII, were isolated by two-dimensional gel electrophoresis and further confirmed by N-terminal sequencing. The polyclonal antibodies prepared against rice RCAI and RCAII were found to cross-react with two RCA polypeptides present in leaf extracts of spinach and tobacco. Furthermore, two different 3' ends of rca mRNA were detected by reverse transcription-polymerase chain reaction analysis. These cDNA fragments and the related genomic DNA fragment were cloned and sequenced. The sequence of rcaI is almost identical to the corresponding sequence of rcaII, except for its having 33 additional amino acids at the C-terminal portion. It can be concluded that a novel alternative splicing mechanism for a common rca mRNA precursor near the 3' end exists in rice plants.

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An electroporation-mediated method for the study of foreign gene expression within chloroplasts has been developed. The chloroplast expression vector pHD203-GUS, which consists of coding regions for beta-glucuronidase (GUS) and chloramphenicol acetyltransferase (CAT) separated by a double psbA promoter fragment from pea (in opposite orientation) was electroporated into spinach chloroplasts and the transient gene expression was examined. Conditions for the expression of the reporter genes have been optimized. Both CAT and GUS activities were detected in chloroplasts electroporated with pHD203-GUS, but not with nuclear expression vector pBI221 or negative control pUC18. No GUS activity was detected when pHD203-GUS was electroporated into spinach protoplasts. Dot immunoblot analysis using anti-GUS antibody confirmed the existence of GUS protein in chloroplasts electroporated with chloroplast-specific vector but not the negative controls, excluding the possibilities of endogenous GUS or bacterial contamination. The expression of GUS protein in treated chloroplasts was further confirmed by Western blot analysis.

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