In testis, several RNA binding proteins have been shown to play a role in t
he translational regulation of specific transcripts. The human protein TRBP
(TAR RNA binding protein) is the homologue of the mouse Prbp (Prm-l RNA bi
nding protein) involved in the protamine mRNA translational delay. TRBP is
known to activate the HIV-1 long terminal repeat but this protein has never
been investigated during spermatogenesis. The aim of this work was to anal
yse the TRBP expression in human testis. By Northern blot analysis, we demo
nstrated a major 1.5 kb transcript present at a high level in human testis
and, to a lesser extent, in some other tissues. In-situ hybridization revea
led that this transcript was present only in elongating spermatids. Antibod
ies raised against a 27 amino acid TRBP-specific peptide revealed a single
protein of 43 kDa expressed in the cytoplasm of elongated spermatids, At th
e ultrastructural level, quantitative analysis of both TRBP mRNA and protei
n, using electron microscopy in-situ hybridization and immunocytochemistry,
showed that TRBP is expressed mainly in spermatids at steps 3-4 of spermio
genesis. These results are in agreement,vith the probable role of TRBP in t
he control of human protamine mRNA translation.