Testis-specific expression of the rat histone Hlt gene in transgenic mice

The testis-specific histone Hlt gene is transcribed only in testis. The appearance of testis-specific nuclear proteins that bind to a unique promoter sequence element designated Hlt/TE located between the H1/AC box and the H1/CCAAT box correlates with the onset of transcription of the Hlt gene during the meiotic cell cycle. In order to determine whether sequences flanking the rat Hlt gene are sufficient to confer tissue-specific expression in vivo, a 6859 bp EcoRI restriction fragment of genomic DNA containing the rat histone Hlt gene has been microinjected into mouse embryos. S1 nuclease protection analysis has shown that the descendants of the resulting transgenic mice express the rat gene in the proper tissue and at the proper meiotic cell cycle stage. Furthermore, when populations of mouse testis cells were prepared by centrifugal elutriation, only the fraction enriched in pachytene primary spermatocytes had a significant steady-state level of rat Hlt mRNA. Although the copy-number of the transgene was variable in these animals, rat Hlt mRNA levels in high copy-number animals never exceeded 2.6 times the level in normal rat testes. The appearance of appropriate meiotic cell cycle-specific transcription indicates the importance of the conserved promoter sequence elements between the two species.