Tracking of sibling fates reliably identifies the differentiative potential of a single cell taken for PCR analysis, and demonstrates the existence of a variety of distinct and stable states of differentiative commitment. Global amplification of cDNA from single precursor cells, identified by sibling fates, yields a true representation of lineage- and stage-specific gene expression, as confirmed by hybridization to a broad panel of probes. The results provide the first expression mapping of these genes that distinguishes between progenitors in different commitment states, generate new insights and predictions relevant to mechanism, and introduce a powerful set of tools for unravelling the genetic basis of lineage divergence.

Stimulation of in vitro transcription mediated by the upstream element of the adenovirus-2 major late promoter (Ad2MLP) involves its recognition by a specific trans-acting factor present in a HeLa whole-cell extract. DNase I footprinting and dimethylsulfate methylation protection experiments were used to determine, at the nucleotide level, upstream sequences which interact with this transcription factor. The ability of upstream element mutants to bind the transcription factor correlates directly with the efficiency of transcription from the corresponding Ad2ML promoters in vivo and in vitro. Competition footprinting experiments show that the transcription factor, which binds to the upstream element of the Ad2MLP, can also interact, but with a lower affinity, with the upstream elements of the Ad2E2a and rabbit ,-Bglobin promoters, both of which display some sequence homology to the 'interacting' region of the Ad2MLP upstream element. The transcription factor does not, however, interact with the upstream elements of either the Ad2E2L, Ad5E3, SV40 early, herpes virus thymidine kinase or chicken conalbumin promoters.

We have previously reported that two fractions derived from HeLa cell S100 extracts, the heparin flow-through and the heparin 0.6 M KCI eluate are required in vitro for efficient and accurate transcription by RNA polymerase class B (II). We have further purified a factor present in the heparin flowthrough fraction, which markedly stimulates specific transcription catalyzed by the heparin 0.6 M KCI eluate. We report here that some of the properties of the stimulatory factor present in our most purified fractions are strikingly similar to those of actin. We demonstrate also that this factor acts at the pre-initiation level of the transcription reaction.

Stimulation of in vitro transcription by the upstream element (UE) of the adenovirus‐2 major late promoter (Ad2MLP) involves a specific trans‐acting factor present in a HeLa whole‐cell extract. By following its transcriptional stimulatory activity and its DNase I footprint on the Ad2MLP‐UE, we have purified this factor to greater than 10% purity and separated it from RNA polymerase B and the general transcription factors required for transcription from the Ad2MLP.