THE ARABIDOPSIS-THALIANA ACT4 ACT12 ACTIN GENE SUBCLASS IS STRONGLY EXPRESSED THROUGHOUT POLLEN DEVELOPMENT/

Plants contain complex actin gene families composed of several diverse and ancient subclasses of genes. One Arabidopsis actin gene subclass represented by the ACT4 and ACT12 genes has been isolated and characte rized. Both actin genes have typical plant actin gene structures, incl uding three small introns interrupting the coding region and an intron within the mRNA leader. Their encoded proteins differ from each other in only one amino acid, whereas they differ in 3-10% of their amino a cids from the other five Arabidopsis actin subclasses. They also share a few small blocks of DNA sequence homology in the 5' flanking region near their TATA boxs, but not in their introns, 3' flanking regions, or degenerate positions within codons. Southern analysis with gene-spe cific probes from 5' flanking sequences showed that both were single c opy genes in the genome. Both RNA gel blot analysis with 3' gene-speci fic probes and reverse transcriptase-mediated polymerase chain reactio ns (RT-PCR) with gene-specific primers detected low levels of ACT4 and ACT12 mRNAs in flowers and very high levels in pollen. The RT-PCR det ected very low levels of these mRNAs in the vegetative organs. The 5' region from both genes, including the promoter region, TATA box, the s equence for the mRNA leader and its intron, and the first 19 actin cod ons, was fused to a beta-glucuronidase (GUS) reporter gene. Expression of the GUS fusions were examined histochemically in 40 independent tr ansgenic Arabidopsis plants. Expression of the ACT4/GUS fusion was res tricted to young vascular tissues, tapetum, and developing and mature pollen. Similar expression patterns in these tissues and cell types we re observed for ACT12/ GUS fusion, yet unlike ACT4, ACT12 was also str ongly expressed in the root cap and in a ring of pericycle tissues dur ing lateral root initiation and early development. The unique expressi on patterns of the ACT4/ACT12 actin gene subclass are discussed in lig ht of recent data on the other expressed members of the Arabidopsis ac tin gene family.