Threonine production by co-existence of cloned genes coding homoserine dehydrogenase and homoserine kinase in Brevibacterium lactofermentum.

Abstract

The homoserine kinase (HK) gene was cloned from Brevibacterium lactofermentum M-15, a threonine-producing mutant. Then, a trimethoprim-resistant hybrid plasmid pAJ212 expressing the HK gene was constructed. A threonine-producing recombinant strain harboring pAJ210, a chloramphenicol-resistant plasmid on which homoserine dehydrogenase (HD) gene was cloned, was transformed further with the obtained HK-expressing plasmid pAJ212. Plasmids pAJ210 and pAJ212 were compatible in B. lactofermentum because these two plasmids were derived from two different cryptic plasmids, pHM1519 and pAM330 respectively. Co-existence of the cloned HD and HK genes in M-15 resulted in more than a 30% increase of threonine production (33 g/liter) and a decrease of the by-products (lysine and homoserine) as compared with M-15 carrying HD recombinant plasmid alone. The activities of HD and HK in the strain harboring both HD- and HK-recombinant plasmids were elevated markedly by gene dosage effect, which brought about the apparent increase in threonine production.