Microbial hosts engineered for increased tolerance to temperature shifts

The present invention relates to microbial host cells that have been engineered for increased tolerance to temperature shifts, for increased performance at temperatures different from the microorganism's optimal temperature and/or for changing at least one of the microorganism's cardinal temperatures by replacing an endogenous NAD+ biosynthesis gene by a heterologous gene encoding a corresponding enzyme with another temperature profile and/or from a microorganism with a different optimum growth temperature. The invention further relates to processes wherein the engineered microbial host cells are used for producing a fermentation product, and to the use nucleotide sequences encoding NAD+ biosynthesis gene for changing at least one of a microorganism's cardinal temperatures and/or for improving a microorganism's tolerance to temperature shifts.