NOVELTY - System for decreasing a cellular growth rate, has a host cell comprising a first nucleotide sequence encoding an mRNA interferase operably linked to a first heterologous regulatory element, where the expression of the first nucleotide sequence encoding mRNA interferase in the host cell diminishes the growth rate but does not arrest cellular growth completely.    USE - The system is useful for decreasing a cellular growth rate, decreasing accumulation of toxic metabolites during fermentation, decreasing oxygen consumption during fermentation, or decreasing heat generation during fermentation (all claimed), for providing new fermentation technologies and optimizing existing fermentation technologies for producing biopharmaceuticals e.g. insulin, enzymes e.g. tannase, antibiotics e.g. chloramphenicol, biosurfactants e.g. glycolipids, biological fuels e.g. biodiesel, amino acids e.g. L-lysine, organic acids e.g. lactic acids, polyols e.g. glycerol, vitamins e.g. vitamin B2, pigments e.g. leucopene, and biopolymers and plastics e.g. 1,3-propanediol. Can also be used in strain development and metabolic engineering.    ADVANTAGE - The production of targeted cellular metabolites can be enhanced. The accumulation of undesired by-products can be decreased, thus the industrial fermentation process can be increased effectively and economically.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method of (1) decreasing a cellular growth rate, (2) decreasing accumulation of toxic metabolites during fermentation, (3) decreasing oxygen consumption during fermentation, or (4) decreasing heat generation during fermentation, which involves (a) providing a host cell, (b) cloning a first nucleotide sequence encoding an mRNA interferase, (c) operably linking the first nucleotide sequence encoding mRNA interferase to a first heterologous regulatory element, and (d) expressing the first nucleotide sequence encoding mRNA interferase operably linked to the first heterologous regulatory element in the host cell, where the expression of the first nucleotide sequence encoding mRNA interferase in the host cell diminishes the (i) cellular growth rate but does not arrest cellular growth completely, (ii) accumulation of the toxic metabolites during the fermentation, (iii) oxygen consumption during the fermentation, or (iv) heat generation during the fermentation, respectively.