Of Fingers, Zippers, and Boxes

IN THlS ISSUE Transcription in eukaryotic cells is controlled by a large number of transacting regulatory proteins, most of which recognize cis-acting DNA sequences 5' to the transcription start site. In several cases, it has been demonstrated that expression and specific DNA binding of transcriptional regulators in the context of a developmental program control the temporal and spatial expression of genes. Transcriptional regulators are also part of intracellular sig-na1 transduction chains by which cells integrate environmental and physiological cues to change the transcriptional status of genes. As an example, light is one of the most widely studied signals in plants, and several proteins have been identified that bind to regulatory DNA sequences in the promoter regions of light-responsive genes (Gilmartin et al., 1990). The biochemical pathways and the genes downstream of the transcriptional regulators, however, are still largely unknown. The isolation of transcriptional regulators and their genes and the identification of their DNA binding sites are often the important first steps in the genetic, biochemical, and molecular dissection of signal transduction chains. In this respect, plants are no exception , and the efficient transformation systems available for many plants are, in fact, powerful tools that allow the rapid and detailed dissection of regulatory promoter elements. Two reports in this issue analyze promoter regions for genes that have complex developmental and organ-specific expression patterns. Ohl et al. (pages 837-848) describe the isolation and partia1 characterization of a gene for phenylalanine ammonia-lyase (PAL) from Arabidopsis. This enzyme catalyzes the first step in the biosynthesis of phenylpropanoids, which are synthesized in response to the plant developmental program, pathogen attack, and UV and mechanical stress. In Arabidopsis, PAL is encoded by a multigene family of comparable size to those from bean, parsley, and rice, supporting the hypothesis that individual PAL genes may encode variant products with distinct functional specializations. Expression of p-glucu-ronidase activity from the PAL promoter in transgenic Arabidopsis responds to developmental cues, wounding, and light. This expression pattern is similar to the response of the bean PAL promoter in transgenic tobacco (Bevan et al., 1989; Liang et al., 1989) and suggests that regulatory elements within the promoter regions are conserved in a wide range of plants. Consistent with this is the identification in Ohl et al. (pages 837-848) and Lois et al. (1989) of DNA sequence motifs that are conserved in the Arabidopsis and parsley PAL promoter regions and that are present …