Analysis of the structural and functional properties of chromosomal DNA fragments of Streptococcus thermophilus ST128 delineated the promoter sequence STP2301 and identified its -35, -10 and Shine-Dalgarno regions. STP2201 was used in cloning vectors derived from small resident plasmids pER8 (2094 bp) and pER371 (2672 bp) of S. thermophilus strains to facilitate expression of a Streptomyces sp, marker gene (cholesterol oxidase) in lactic acid bacteria, Cell extracts of ST128 transformants converted up to 75% of cholesterol into 4-cholesten-3-one during a h of incubation.