The actin multigene family in soybean contains at least six members. These plant actins accumulate amino acid substitutions at a rate similar to the animal actins and yet there is far greater diversity between kappa, lambda, and mu soybean actins than between mammalian cytoplasmic and muscle actins. The steady state mRNA levels produced from these diverse soybean actin genes varies over a large range and shows some organ specificity. A variety of cytoplasmic functions and levels of expression are known for both animal and plant actin proteins both within cells and in different tissues. Therefore, it is proposed that the diverse plant actin genes are under tissue-specific regulation producing actins for a variety of functions. In this proposal, extensive data on plant actin gene sequence, predicted protein sequence, and structure of the transcription unit is used to address this hypothesis. A variety of antibodies, including those to synthetic peptides from the kappa, lambda, and mu soybean actins, will be used to localize the three classes of actin to different tissue types and stages of development by immunocytochemical methods. Differential gene expression will be confirmed by the construction of chimeric actin genes and examining their function in transgenic petunia plants. This includes chimeras between soybean, petunia, and maize actin genes and non-actin gene fusions. Gene, promoter, and transcript structure studies will continue on all of the soybean and petunia actins. This study should identify unique tissue-specific DNA regulatory elements and provide new insights into actin function in plants. The functions of cytoplasmic actin and other cytoskeletal proteins in animals are complex and only recently coming to light. Considering the ubiquitous nature of actin in all eukaryotes, it is likely that research on the diverse function and expression of plant actins will have a striking impact on the study of cytoskeletal structure in general.
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