Date of Award:

2003

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Nutrition and Food Sciences

Advisor/Chair:

Bart C. Weimer

Abstract

Since the catabolism of amino acids in cheese results in the formation of most volatile flavor compounds, a proper intracellular pool of amino acids must be established in order to produce a desirable flavor production in cheese. Generation of this pool of amino acids requires complex interactions among casein and its derivatives, proteolytic enzymes, and transport systems in the associated bacteria, including lactococci. In this project, we hypothesized that casein hydrolysis by the extracellular proteinases of Brevibacterium linens BL2 modulates the expression profile of proteolytic related genes in Lactococcus lactis spp. lactis IL1403.

In order to monitor the global gene regulation patterns in L. lactis ssp. lactis IL1403, a high-throughput gene expression tool was needed to study the gene expression profiles on a genomic scale. In this project, we developed a novel oligonucleotide-based filter DNA array protocol for this purpose. The success of this oligonucleotide-based DNA array was dependent on technical innovations including polyI tailing, indirect high density biotin labeling, careful probe design, and integrated computational data analysis. The utility and validity of this protocol were demonstrated by profiling the expression of 375 metabolically related genes in L. lactis ssp. lactis IL1403 during heat, acid, and osmotic stresses.

Subsequently the DNA macroarray was used to profile the gene expression changes of L. lactis spp. lactis IL1403 growing in a peptide-limited medium, in a casitone-based peptide-rich medium, and in a casein hydrolyte by B. linens BL2 proteolytic enzymes. L. lactis ssp. lactis IL1403 experienced nitrogen starvation even with an abundance of peptide resources because of lack of expression of peptide transporter genes. Conversely, a peptide pool generated by B. linens BL2 proteolytic activities was sufficient to sustain the growth of L. lactis ssp. lactis IL1403. The repression of the peptide transporter and other peptidase genes of L. lactis ssp. lactis IL1403 was relieved in this medium. Interestingly, the Opt system, a di-tripeptide transporter, was used as a primary peptide transporter, instead of the Opp system whose genes were not actively transcripted in IL1403.

We also conducted additional experiments to further describe the protease in B. linens BL2 responsible for the peptide pool generation. This enzyme was secreted as a non-active zymogen and matured into the active protease. Both proteolysis and maturation processes were regulated. Collectively, this work demonstrated that a unique protease of B. linens BL2 generated a pool of pep tides transportable by L. lactis IL1403 and induced changes in gene expression in L. lactis IL1403. Consequently, this body of work demonstrated the hypothesis to be true.

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