Diverse soybean actin transcripts contain a large intron in the 5' untranslated leader: structural similarity to vertebrate muscle actin genes

Abstract

Plant actins are encoded by complex and highly divergent multigene families. Despite the general lack of intron conservation in animal, fungal and protist actin genes, evidence is presented which indicates that higher plant actin genes have an untranslated leader exon with structural similarity to that found in vertebrate actin genes. All functional higher plant actin genes sequenced to date contain a potential intron acceptor site in the 5' untranslated region 10 to 13 nucleotides upstream of the initiator ATG. A leader specific cDNA probe hybridized to sequences over 1.0 kbp upstream from the coding region confirming the presence of an upstream exon. Primer extension of mRNA with gene-specific oligonucleotides was used to analyze the 5' untranslated exon and leader intron from four divergent soybean actin genes, SAc3, 4, 6 and 7. The 5' ends of all four mRNAs are heterogeneous. The consensus promoter elements of the SAc7 actin promoter were identified. Gene specific primer extension sequencing of actin mRNAs indicated that splicing of the 5' leader intron occurred at the predicted acceptor site in SAc6 and SAc7. The SAc6 and SAc7 5' untranslated exons are small (88-111 nt) and the leader introns are relatively large (844-1496 nt). The presence of an intron within the 5' RNA leader and an intron which splits a glycine codon at position 152 in all plant actin genes and all vertebrate muscle actin genes suggests that these structures may have been conserved due to a functional role in actin expression. The 5' regions of these two soybean actin genes contain many unusual features including (CT) repeats and long stretches of pyrimidine-rich DNA. The possible roles of the upstream exon/intron and the C + T-rich regions are discussed.