The present invention relates to beta-ketoacyl ACP synthase genes of the KASI/KASIV type and proteins encoded by these genes. The genes can be included in nucleic acid constructs, vectors or host cells. Expression of the gene products can alter the fatty acid profile of host cells. The KAS genes can be combined with a FATA or FATB thioesterase gene to create a cell that produces an increased amount of C8-C16 fatty acids. Suitable host cells include plastidic cells of plants or microalgae. Oleaginous microalga host cells with the new genes are disclosed.