Population Council Research that makes a difference

Initiation of mRNA synthesis is a key regulatory step in the control of protein-encoding gene expression. Initiation is mediated by RNA polymerase II and a surprisingly complex array of general initiation factors (TFIIA, -B, -D, -E, -F, and -H), which recognize the core promoters of specific genes; these factors are highly conserved from yeast to man. The discovery of TATA-binding protein (TBP)-related factors (TRFs) and TBP-associated factors (TAFs) has suggested alternative mechanisms for gene-specific transcriptional regulation and raised interest in their biological functions in specific cell types. These initiation factors are multiprotein complexes that serve to bridge RNA polymerase II with specific transcription factors; each complex consists of the TBP and several TAFs (TAFIIs).

Human TBP-related factor 2 (TRF2) is encoded by two mRNAs with common protein coding but distinct 5' nontranslated regions. One mRNA is expressed ubiquitously (TRF2-mRNA1), whereas the other (TRF2-mRNA2) shows a restricted expression pattern and is extremely abundant in testis. Human TRF2 forms a stable stoichiometric complex with human TFIIA, but not with TAFs. In contrast to recent observations of an embryonic lethal phenotype for TRF2 inactivation in C. elegans and Xenopus, researchers in the Morris laboratory found that TRF2-deficient mice are viable.

However, mice lacking the TRF2 gene, which is highly expressed in the testis, have a severe defect in spermiogenesis (i.e., the process of postmeiotic differentiation to testicular sperm). Postmeiotic round spermatids advance at most to step 7 of differentiation but fail to progress to the elongated form, and gene-specific transcription deficiencies were identified. Researchers showed that the normal expression of TRF2 is both cell type- and stage-specific. In a normal male, TRF2 mRNA is detected in the late pachytene spermatocytes at stage VIII and increases throughout the subsequent stages.

After meiotic divisions, TRF2 expression declines from stage I to stage V concomitantly with the progression of spermiogenesis from round to elongating spermatids. Taken together, the data are consistent with an essential regulatory role of TRF2 in male germ cell differentiation during spermatogenesis. Based on these findings, these researchers propose that mammals may have evolved more specialized TRF2 functions in the testis that involve transcriptional regulation of genes essential for spermiogenesis.