Identification of USF as the ubiquitous murine factor that binds to and stimulates transcription from the immunoglobulin λ2 chain promoter

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Abstract
To study the specificity and identity of NF-λ2, a ubiquitous murine nuclear factor that interacts specifically with the promoter of the λ2-chain gene and stimulates its transcription, competition experiments were carried out using DNA fragments from various immunoglobulin regulatory elements. The results showed that a fragment containing the H-chain enhancer competed efficiently for the bonding of NF-λ2. Dissection of the H-chain enhancer revealed that the μE3 motif contributed the competing ability. Additionally, a regulatory region found in the adenovirus major late promoter, which interacts with the human general transcription factor USF, competed very efficiently for bonding to NF-λ2. This region contains a sequence, CACGTGAC, which is identical to a region within the NF-λ2 motif. The pattern of complexes formation using oligonucleotide probes corresponding to the NF-λ2 and USF motifs were identical, and they both differed from that displayed by the E3 probes. Antisera against different domains of USF also react specifically with NF-λ2 showing that this factor is antigenically related, if not identical, to USF. Furthermore, the activity of the λ2 promoter in an in vitro transcription assay was significantly reduced when the nuclear extract used was USF-depleted. Addition of exogenous USF to this extract restored the transcription activity. Therefore, we conclude that NF-λ2 is the murine homologue of USF.