664 Part VI: Fermented Foods
whereas glucose represses, the transcription of
MALSand MALTgenes (Federoff et al. 1983a,b;
Needleman et al. 1984). The constitutively ex-
pressed regulatory protein (MALR) binds near the
MALS and MALT promotors and mediates the
induction of MALSand MALTtranscription (Cohen
et al. 1984, Chang et al. 1988, Ni and Needleman
1990). Experiments with MALR-disrupted strains
led to the conclusion that MalRp is involved in glu-
cose repression (Goldenthal and Vanoni 1990, Yao et
al.1994). Relatively little attention has been paid to
posttranscriptional control (i.e., the control of trans-
lational efficiency) or mRNA turnover as mecha-
nisms complementing glucose repression (Soler et
al. 1987). The addition of glucose to induced cells
has been reported to cause a 70% increase in the
lability of a mRNA population containing a frag-
ment of MALS(Federoff et al. 1983a). The third lev-
el of control is posttranslational modification. In the
presence of glucose, maltose permease is either re-
versibly converted to a conformational variant with
decreased affinity (Siro and Lövgren 1979, Peinado
and Loureiro-Dias 1986) or irreversibly proteolyti-
cally degraded, depending on the physiological con-
ditions (Lucero et al. 1993, Riballo et al. 1995). The
latter phenomenon is called catabolite inactivation.
Glucose repression is accomplished by the Mig1p
repressor protein, which is encoded by the MIG1
gene (Nehlin and Ronne 1990). It has been shown
that Mig1p represses the transcription of all three
MALgenes by binding upstream of them (Hu et al.
1995). The MIG1gene has been disrupted in a hap-
loid laboratory strain and in an industrial polyploid
strain of S. cerevisiae(Klein et al.1996). In the
MIG1-disrupted haploid strain, glucose repression
was partly alleviated; that is, maltose metabolism
was initiated at higher glucose concentrations than
in the corresponding wild-type strain. In contrast,
the polyploid mig1strain exhibited an even more
stringent glucose control of maltose metabolism
than the corresponding wild-type strain, which could
be explained by a more rigid catabolite inactivation
of maltose permease, affecting the uptake of maltose.
Recently, the gene AGT1, which codes for a -
glucoside transporter, has been characterized (Han
et al. 1995). AGT1is found in many S. cerevisiae
laboratory strains and maps to a naturally occurring,
partially functional allele of the MAL1locus. Agt1p
is a highly hydrophobic, postulated integral mem-
brane protein. It is 57% identical to Mal61p (the
maltose permease encoded at MAL6) and is also a
member of the 12 transmembrane domain superfam-Table 29.2.Carbohydrate Composition of WortsWort Carbohydrate Content
Origin Danish Canadian British
Type of Wort Lager Lager Pale Ale
Original Gravity 1043.0 1054.0 1040.0
Fructose (g/L) 2.1 1.5 3.3
(%)a 2.7 1.6 4.8
Glucose (g/L) 9.1 10.3 10.0
(%)a 11.6 10.9 14.5
Sucrose (g/L) 2.3 4.2 5.3
(%)a 2.9 4.5 7.7
Maltose (g/L) 52.4 60.4 38.9
(%)a 66.6 64.2 56.5
Maltotriose (g/L) 12.8 17.7 11.4
(%)a 16.3 18.8 16.5
Total ferm. sugars(g/L) 78.7 94.1 68.9
Maltotetraose (g/L) 2.6 7.2 2.0
Higher sugars (g/L) 21.3 26.8 25.2
Total dextrins(g/L) 23.9 34.0 25.2
Total sugars (g/L) 102.6 128.1 94.1
Source:Adapted from Hough et al. 1982.
aPercent of the total fermentable sugars.