Fundamental Food Microbiology, Third Edition - Fuad Fathir

GENETICS OF SOME BENEFICIAL TRAITS 169
but in Lab. delbrueckii ssp. bulgaricus and Lab. helveticus they are on the chromosomes.
Whereas in Lac. lactis they are inducible (also in Lab. acidophilus), in
the two Lactobacillus species they are constitutive. Limited studies have revealed
lac genes to be plasmid linked in Leu. lactis and chromosomally linked in Str.
thermophilus.
In the Lac. lactis strains, lactose fermentation usually includes its transportation
by the PEP-PST system as lactose phosphate, hydrolysis by phospho-b-galactosidase
to glucose and galactose-6-phosphate, and conversion of galactose-6-phosphate to
tagatose-6-phosphate and then to two triosephosphates before entering the EMP
pathway (see Chapter 11). Glucose is also converted to triosephosphate in the EMP
pathway. Triosephosphates are finally converted to lactic acid. In Lab. delbrueckii
ssp. bulgaricus, lactose is transported (along with a proton) by lactose permease and
hydrolyzed by b-galactosidase to glucose and galactose. Glucose is metabolized
through the EMP pathway, whereas galactose is first metabolized by the Leloir
pathway to glucose-1-phosphate before entering the EMP pathway. In Str. thermophilus,
following transport of lactose by lactose permease and hydrolysis by bgalactosidase
to glucose and galactose, glucose is metabolized by the EMP pathway;
in most strains, galactose is excreted into the environment.
The plasmid-linked lac genes in Lac. lactis strains have been characterized, and
the function of each gene has been determined. A total of eight genes are arranged
in an operon system and designated as lac ABCDFEGX. The four genes, lac ABCD,
encode the three enzymes in the tagatose-6-phosphate pathway: the lac AB is
involved in the conversion of glucose-6-phosphate to tagatose-6-phosphate, lac C
in the conversion of tagatose-6-phosphate to tagatose-1,6-diphosphate, and lac D in
the conversion of the latter to glyceroldehyde-3-phosphate and dihydroacetone phosphate.
The lac FE genes encode the PEP-PTS and the lac G encodes the hydrolysis
of lactose-6-phosphate. The function of lac X is not known. The promoter of the
operon is located upstream of lac A, and the regulator gene (lac R) is located
immediately upstream of the promoter. It encodes the repressor protein that regulates
the expression of lac operon, which is inducible. The arrangement, location, and
regulation of lac genes in other lactic acid bacteria have not yet been properly
identified. 17,21
The las genes, or the genes encoding for lactic acid synthesis, have been identified
and characterized on the chromosome of Lac. lactis. Five genes are involved
for encoding the enzymes necessary in the process: pfk, for phosphofructokinase;
pyk, for pyruvate kinase; ldh, for L-(+)-lactate dehydrogenase; tpi, for triosephosphate
isomerase, and gap, for glyceraldehyde-3-phosphate dehydrogenase. The three
genes, las pfk, las pyk, and las ldh are organized in an operon (the las operon) with
the promoter located upstream of las pfk and a terminator located downstream of
las ldh. The genes las tpi and las gap are located separately on the chromosome
and each is expressed as a monocistronic gene.
The five genes are highly biased in their codon usage, suggesting they are
strongly expressed in Lac. lactis. Limited results have indicated that the expression
of the three genes in the las operon is under a genetic regulatory mechanism, which
limits their expression and the level of lactic acid production. 17,21
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