Improvement of Escherichia coli strains overproducing lysine using recombinant DNA techniques

Mismatch-CRISPRi Reveals the Co-varying Expression-Fitness Relationships of Essential Genes in Escherichia coli and Bacillus subtilis

Essential genes are the hubs of cellular networks, but lack of high-throughput methods for titrating gene expression has limited our understanding of the fitness landscapes against which their expression levels are optimized. We developed a modified CRISPRi system leveraging the predictable reduction in efficacy of imperfectly matched sgRNAs to generate defined levels of CRISPRi activity and demonstrated its broad applicability. Using libraries of mismatched sgRNAs predicted to span the full range of knockdown levels, we characterized the expression-fitness relationships of most essential genes in Escherichia coli and Bacillus subtilis. We find that these relationships vary widely from linear to bimodal but are similar within pathways. Notably, despite ∼2 billion years of evolutionary separation between E. coli and B. subtilis, most essential homologs have similar expression-fitness relationships with rare but informative differences. Thus, the expression levels of essential genes may reflect homeostatic or evolutionary constraints shared between the two organisms.

Construction of Lactobacillus plantarum strain with enhanced L-lysine yield

AIM

To enhance L-lysine secretion in Lactobacillus plantarum.

METHODS AND RESULTS

An S-2-aminoethyl-L-cystein (AEC)-resistant mutant of L. plantarum was isolated, and it produced L-lysine at considerably higher level than the parent strain. Aspartokinase in the mutant has been desensitized to feedback inhibition by L-lysine. The nucleotide sequence analysis of thrA2 that codes for aspartokinase in the mutant predicted a substitution of glutamine to histidine at position 421. L-Lysine-insensitive aspartokinase, together with aspartate semialdehyde dehydrogenase, dihydrodipicolinate synthase, and dihydrodipicolinate reductase genes, was cloned from L. plantarum DNA to a shuttle vector, pRN14, and the genes were then transformed individually into the AEC-resistant mutant and the parent strain. The overexpression of the genes led to the increase in the activity of enzymes they encode in vitro. However, only the strain overexpressing aspartokinase or dihydrodipicolinate synthase produced more L-lysine.

CONCLUSIONS

The desensitization of aspartokinase to L-lysine in L. plantarum led to the overproduction of L-lysine. The overexpression of L-lysine-insensitive aspartokinase or dihydrodipicolinate synthase enhanced L-lysine secretion in L. plantarum.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of the L-lysine-overproducing strain of L. plantarum in food or feed fermentation may increase the L-lysine content of fermented products.

Evaluation of lysine and methionine production in some Lactobacilli and yeasts from ogi

Lysine and methionine producing cultures of Lactobacillus and yeasts in batch fermentation of ogi were selected by growth in the presence of the analogues, S-2-aminoethyl]-L-cysteine (thialysine) and ethionine respectively. The study shows that 42.5% of the Lactobacillus and 83.3% of the yeast isolates tested were capable of lysine production while 25.0% of the Lactobacillus and 87.8% of the yeast isolates produced methionine. The lysine and methionine yields of Lactobacillus were significantly (P <0.01) higher than that of yeasts. The majority of the yeast isolates excreted most of the lysine and methionine produced. More lysine was produced than methionine in all tested isolates.

Chromosomal location and nucleotide sequence of the Escherichia coli dapA gene

In Escherichia coli, the first enzyme of the diaminopimelate and lysine pathway is dihydrodipicolinate synthetase, which is feedback-inhibited by lysine and encoded by the dapA gene. The location of the dapA gene on the bacterial chromosome has been determined accurately with respect to the neighboring purC and dapE genes. The complete nucleotide sequence and the transcriptional start of the dapA gene were determined. The results show that dapA consists of a single cistron encoding a 292-amino acid polypeptide of 31,372 daltons.

Cloning and expression in Escherichia coli of genes involved in the lysine pathway of Brevibacterium lactofermentum

The Brevibacterium lactofermentum genes which complement Escherichia coli lysA and asd-1 mutants were identified, respectively, as a 1.9-kilobase PstI-ClaI fragment and a 2.5-kilobase PstI fragment by cloning into pBR325. Southern blot transfers show hybridization to chromosomal fragments of identical size. The putative B. lactofermentum asd and lysA products are 44 and 48 kilodaltons, respectively.