Genetics of Some Beneficial Traits ◾ 155
and hydrolyzed by β-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 β-galactosidase 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 lacABCDFEGX. Four genes, lacABCD, encode the three enzymes in the tagatose-6phosphate pathway: The lacAB is involved in the conversion of glucose-6-phosphate to tagatose-6phosphate, lacC in the conversion of tagatose-6-phosphate to tagatose-1,6-diphosphate, and lacD
in the conversion of the latter to glyceroldehyde-3-phosphate and dihydroacetone phosphate. The
lacFE genes encode the PEP-PTS, and the lacG encodes the hydrolysis of lactose-6-phosphate.
The function of lacX is not known. The promoter of the operon is located upstream of lacA, and
the regulator gene (lacR) 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.
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 that 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.
Conclusion
The importance of many strains of lactic acid bacteria in the fermentation of diverse types of food,
production of different food additives, use as probiotics, and application to deliver drugs has necessitated the understanding of the molecular basis of these desirable characteristics. With a modest
beginning in early 1970s, research activities on the genetics of lactic acid bacteria have exploded.
Currently, studies on the genome sequence of lactic acid bacteria are generating important information on the fluid nature of the DNA that can change with environment. Comparative genome
analysis, with more information, will enable researchers to determine the minimum genetic information and the arrangements of genetic codes necessary to design a unique starter-culture strain
for a specific use. Production of starter cultures and problems associated with bacteriophages are
discussed in Chapter 14.
QUESTIONS
1. Discuss the important characteristics of plasmids in lactic acid bacteria.
2. Discuss the characteristic differences between large and small plasmids in lactic acid bacteria.
3. List five commercially important phenotypes that are plasmid linked in lactic acid bacteria
and discuss their importance in relation to plasmid stability.
and hydrolyzed by β-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 β-galactosidase 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 lacABCDFEGX. Four genes, lacABCD, encode the three enzymes in the tagatose-6phosphate pathway: The lacAB is involved in the conversion of glucose-6-phosphate to tagatose-6phosphate, lacC in the conversion of tagatose-6-phosphate to tagatose-1,6-diphosphate, and lacD
in the conversion of the latter to glyceroldehyde-3-phosphate and dihydroacetone phosphate. The
lacFE genes encode the PEP-PTS, and the lacG encodes the hydrolysis of lactose-6-phosphate.
The function of lacX is not known. The promoter of the operon is located upstream of lacA, and
the regulator gene (lacR) 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.
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 that 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.
Conclusion
The importance of many strains of lactic acid bacteria in the fermentation of diverse types of food,
production of different food additives, use as probiotics, and application to deliver drugs has necessitated the understanding of the molecular basis of these desirable characteristics. With a modest
beginning in early 1970s, research activities on the genetics of lactic acid bacteria have exploded.
Currently, studies on the genome sequence of lactic acid bacteria are generating important information on the fluid nature of the DNA that can change with environment. Comparative genome
analysis, with more information, will enable researchers to determine the minimum genetic information and the arrangements of genetic codes necessary to design a unique starter-culture strain
for a specific use. Production of starter cultures and problems associated with bacteriophages are
discussed in Chapter 14.
QUESTIONS
1. Discuss the important characteristics of plasmids in lactic acid bacteria.
2. Discuss the characteristic differences between large and small plasmids in lactic acid bacteria.
3. List five commercially important phenotypes that are plasmid linked in lactic acid bacteria
and discuss their importance in relation to plasmid stability.