Abstract: The claimed invention provides a eukaryotic plasmid vector which upon expression provides the enzymes required for cysteine biosynthesis, the vector preferably allowing for expression of these enzymes and concomitant production of cysteine in ruminal mucosa cells. The vector of the instant invention is engineered using recombinant techniques to insert microbial genes encoding serine acetyltransferase (SAT) and O-acetylserine sulphydrolase (OASS), preferably the isolated Salmonella typhimurium genes cysE encoding for SAT and cysK or cysM encoding for OASS, driven by a promoter efficient in a eukaryotic host cell, such as the SV40 late promoter, into a eukaryotic cloning vector, such as Promega's pGEM-2. The construction of the instant vector also allows for an additional gene which upon expression produces human growth hormone.