Mosaic Expression of LacZ Reporter Gene Controlled by 5'-Regulatory Sequences of alpha-S 1 -Casein Gene in Transgenic Mice

The phenomenon of mosaic expression at the cellular level is frequently observed in tissues and organs of transgenic animals. The report concerns mosaicism in the progeny of five transgenic mouse founders carrying the LacZ reporter gene under the control of 5'- regulatory sequences of bovine and goat alpha-S 1 - casein genes of various sizes. Cells positive for β -galactosidase E. coli activity were detected in lactating mam- mary glands of all transgenic females; however, the distribution of positive cells within the mammary glands was variable. We observed two types of mosaics, i.e., lobular (clonal) variegation when most or all lobular cells were positive for β -galactosidase and stochastic mosaicism when only single β -galactosidase positive cells were scattered within mammary glands. It is suggested that the stochastic type of mosaicism is realized in cells at the terminal stage of the differentiation of lactating glands, whereas the lobular one is developed from pro- liferating precursors capable of forming a whole lobule. The ectopic expression of the reporter gene was detected in the mandibular salivary gland in the offspring of two of the five founders No16 and No37, as well as in ovary follicles at the atrezia stage in the progeny of one of these founders. The low level of ectopic expres- sion means that the 5`-flanked regulatory sequences of alpha-S 1 -casein gene of various lengths used in the con- structs ensure the reliable tissue-specific expression of the reporter gene.