Hint: Pseudomonas denitrificans is a bacterium which uses products of its cob gene to synthesize methionine amino acid.
Complete answer: Pseudomonas denitrificans is a gram-negative, heterotrophic bacterium. From the name of the bacterium it is clear to understand that it helps in the denitrification step of the nitrogen cycle. It is aerobic in nature and capable of synthesizing vitamin B12 in aerobic conditions.
Vitamin B12 is also known as cobalamin or cyanocobalamin. It is the product of cob genes. With the use of these genes, the bacterium produces cobalamin in excess quantity i.e. overproduces it for the synthesis of methionine. This pathway is known as the cobalamin-dependent pathway of methionine synthesis.
Hence, scientists found a way to produce vitamin B12 in large quantities by utilizing the principles of genetic engineering. Some of the cob genes were made to overexpress themselves in a particular strain of Pseudomonas denitrificans.
Additional information: This experiment led to the use of this bacterium for large scale production of cobalamin and hence for industrial use. Moreover, this species is more preferred because it produces a higher quantity of the vitamin compared to other cobalamin-producing microbe species.
So, the correct answer is option B12
Note: Vitamin B12 is very essential for good health. It is widely used in the medical and pharmaceutical industry. There are several other microbes that are used in the industrial production of this vitamin.
Example: Propionibacterium shermanii Sinorhizobium meliloti etc.