Serine/Threonine Response Elements

T RANSCRIPTION in eukaryotes involves transcriptional activators, proteins that bind to specific sites distal from the TATA box termed enhancers or u p stream activation sequences (UASs) . When bound to a UAS, a transcriptional activator is able to stimulate the transcription initiation complex leading to synthesis of the mRNA. The ability of Saccharomyces cermisiar to use serine or threonine as the sole nitrogen source depends on the CHAl gene, which encodes the catabolic L-serine (L-threonine) dehydratase (WOS and WIAME 1982; BORNES et al. 1992). CHAl is regulated by transcriptional induction by serine or threonine (PETERSEN et al. 1988). Until now no trawacting factor involved in transcriptional regulation has been identified. Recently, a deletion analysis of the CHAl promoter identified two elements, UASlc;HA and UAS2cm, each of which is sufficient to confer serine and threonine induction to yeast genes (BORNKS et al. 1993). It was also found that the multifunctional protein ABFl binds to an element in the CHAl promoter, irrespectively of CHAl induction. Protein binding to either UASl,:, or UAS2(:13A was not detected using nuclear prctein extracts prepared from cells grown in uninducing or inducing media (BORNKS et al. 1993). However, the regulated expression of CHAl would be expected to involve a transcriptional regulator(s) that, directly or indirectly, senses the presence or absence of serine/threcnine in the cell.