An isolated protein which is at least partially encoded by a polynucleotide sequence encoding a novel acyl-CoA: diacylglycerol acyltransferase 1-like gene (PtDGAT1) from the diatom Phaeodactylum tricomutum is provided together with a composition which includes the isolated protein and a transgenic organism transformed by a polynucleotide encoding same. The invention also provides a method for producing or enhancing the production of oil or triacylglycerols with high saturated fatty acids content.