EXPRESSION OF CEL2 AND CEL4, 2 PROTEINS FROM AGARICUS-BISPORUS WITH SIMILARITY TO FUNGAL CELLOBIOHYDROLASE-I AND BETA-MANNANASE, RESPECTIVELY, IS REGULATED BY THE CARBON SOURCE

Two new cellulose-growth specific (cel) cDNAs, cel2 and cel4, have bee n isolated from an Agaricus bisporus cDNA expression library by immuno screening with an A. bisporus anti'endoglucanase' antibody. The deduce d amino acid sequences showed that both CEL2 and CEL4 proteins have a modular structure consisting of a fungal-type cellulose-binding domain (CBD) and a catalytic domain separated by a linker region rich in Pro , Ser and Thr. The CEL2 and CEL4 catalytic domains were homologous to fungal cellobiohydrolases (CBH) in family 7 and to fungal mannanases i n family 5 of the glycosyl hydrolases, respectively. A previously isol ated cDNA derived from a constitutive gene was also sequenced. The ded uced amino acid sequence corresponded to 5-aminolaevulinic acid syntha se (ALA), the first enzyme in the haem biosynthetic pathway, and was m ost similar to other fungal ALAs. RNA analysis showed that the express ion of cel2 and cel4 genes was induced by cellulose and repressed by g lucose, fructose and lactose. The soluble cellulose derivative CM-cell ulose induced mRNA accumulation for cel1 but not cel2, cel3 or cel4. M annitol, maltose, sorbitol and glycerol decreased cel2 and cel4 mRNA l evels to different extents. cel1, cel2, cel3 and cel4 mRNAs all disapp eared after the addition of glucose with apparent half-lives of less t han 20 min. Whether cel mRNAs have short half-lives or glucose affects the stability of cel transcripts remains to be investigated.