DIFFERENTIAL EXPRESSION OF SUC GENES - A QUESTION OF BASES

Non-coding nucleotide sequences located 5' upstream of the transcripti onal start site play an essential role in gene expression as they cont ain binding sites for transcription and regulatory factors. The yeast SUC gene family is a useful model to study the influence that nucleoti de exchanges within the promoter regions have on their expression, sin ce (i) these genes, regulated by glucose repression, are differentiall y transcribed (invertase activity produced by distinct SUC genes may s how variations of about 10-fold); and (ii) promoter sequences of SUC3, SUC4, SUC5 and SUC7 are more than 99% homologous, showing only six ba se exchanges among all of them. Comparison of these nucleotide exchang es with the expression of each SUC gene (located either on chromosomes or on multicopy and centromeric plasmids) points out that naturally o ccurring base exchanges as few as one nucleotide modification (G to A transition at position - 497 relative to the translational start site, C to T transition at position - 460 and insertion/deletion of a T at positions - 590, - 586 and - 435) may have a strong effect on gene exp ression.