We have cloned and expressed in Escherichia coli a section of the Moloney murine leukemia virus (Mo-MLV) pol gene which includes the entire coding region of mature reverse transcriptase (RT) plus 284 additional base pairs 3' to the coding region (Kotewicz et al., 1985). To prepare cloned Mo-MLV RT as close as possible to authentic RT in structure and activity, the universal terminator sequence GC(TTAA)3GC was introduced at a number of positions inside and outside the RT coding region within 200 nucleotides of its 3' end. The level of RT activity expressed from these constructs varied sevenfold. This variation was found to be directly related to the stability of the RT protein products in the E. coli K-12 strain K802; half-lives varied from 2 to 35 min. The stability of most of the RT proteins was not increased in E. coli K802 lon- cells, with the exception of two, whose half-lives were increased by a factor of two.