PARTICIPATION OF CHROMATIN IN THE REGULATION OF PHASEOLIN GENE-EXPRESSION

It has long been recognized that multiple transcription factors contri bute to the overall regulation of gene expression. Since it is inconce ivable that a specific transcription factor is assigned to each protei n-encoding gene, it has been suggested that several factors interact i n various combinations (Goldberg, 1988; Thomas, 1993). However, it is not clear if such combinations are established by the relative linear positions of the corresponding cis-element, by the order in which bind ing sites are occupied (providing alternative surfaces for protein-pro tein interactions), or by both of these parameters, possibly in associ ation with other influences. One of these influences is undoubtedly ch romatin. While this has previously considered being only a repressive structure, the recent finding that several transcriptional regulators can acetylate histones is rapidly leading to the conclusion that modif ication of histones, and thus chromatin, is a vital step in activation of the transcriptional machinery (Wade and Wolffe, 1997). Evidence fo r replacement of the repressive nucleosome structure by TFIID during t ranscription activation has been postulated by Van Holde and Zlatanova (1996) and may well be involved in phas promoter activation. Discerni ng actual steps by which the repressive nucleosome structure is modifi ed or displaced will provide new insight to plant gene regulation, and it is tempting to think that histone acetylation will be part of the key. Once the chromatin structure is relaxed, transcription factors ca n gain access to the promoter, and a careful evaluation of changes in cis-element occupancy during embryogenesis using the sensitive DMS foo tprinting and LMPCR analysis technique will give important clues to th e types of proteins involved. Another future opportunity for these app roaches will be to explore events that lead to the cessation of expres sion as the embryo matures.