The Lactobacillus plantarum alr gene encoding alanine racemase was cloned by complementation of an Escherichia coli Alr- DadX- double mutant strain. Knockout of the alr gene abolished all measurable alanine racemase activity, and the mutant was shown to be strictly dependent on D-alanine for growth.
Hols, P., Defrenne, C., Ferain, T., Derzelle, S., Delplace, B., & Delcour, J. (1997). The alanine racemase gene is essential for growth of Lactobacillus plantarum. Journal of bacteriology, 179(11), 3804-3807. https://hdl.handle.net/2078.5/135589 (Original work published 1997)