The fission yeast Schizosaccharomyces pombe has been utilized as a model system to study transcription factors. Yeast replicating vectors containing HIV-1 LTR-CAT (chloramphenicol acetyl transferase gene) and deletions of the HIV-LTR promoters were transfected in S. pombe cells. In a separate experiment, an SV40 promoter transcribing the TAT gene was used in a cotransfection assay. Our results show that HIV-LTR promoter is functional in yeast and RNA synthesis initiates from two sites, One of the 5' ends of the RNA corresponds to the 5' end of the HIV-mRNA observed in viral cells. Deletion analysis of the promoter suggests NF-kB binding sequences are required for promoter activity. However, the TAT gene product failed to transactivate transcription. A number of yeast replicating vectors have been developed. Enhancers which are generally present on viral promoters, such as SP1, NF-kB, CRE, and AP1, have been shown to be functional in fission yeast. A number of viral promoters have been introduced into the yeast such as the human cytomegalovirus (CMV), human low density lipoprotein receptor (LDL), human chorionic gonadotrophin- alpha (HCG-alpha), alpha-crystallin and albumin. The level of activity of these promoters is dependent on the type of enhancer present upstream. In our attempts to identify what causes the instability of HIV-gag mRNA, we observed that transcription of the HIV gag gene from the SV40 promoter in S. pombe results in a series of mRNAs that are less than full-length. These may result from poly-A signals within the coding region and are candidate crs sequences. Cis regulating sequences are responsible for dependence of HIV mRNAs on rev protein.
