Pw Sox-1: the first member of the Sox gene family in Urodeles.

The Y chromosomal switch gene SRY, which initiates testis development during mammalian embryogenesis was recently described (l).The SRY-encoded protein contains a 79 amino acid DNA-binding motif referred to as an HMG-box (2) known to exist in other transcription factors or proteins involved in DNA bending (3). On the basis of the homology between their HMGboxes, other SRY-related genes have been described in mouse (4, 5), human (5, 6), and non-mammalian organisms such as Drosophila, Xenopus (5), and gull (7). SRY-related genes have been named Sox (for SRY-box). We have identified a new Sox gene from PUurodeles waltl (Urodele, Amphibian) by using PCR amplification of male Pleurodeles DNA with SRY HMG-box primers (forward primer: 5'AAGCGACCCATGAA(C/T)GCNTT, reverse primer: 5'GTACTT(G/A)TA(G/A)T(C/T)NGG(A/G)TA). PCR products were subcloned into a Smal-digested pBluescript SK, and a clone, PwSox-1, was sequenced and compared to other Sox genes (Figure 1). According to Laudet et al. (8), this sequence unambiguously belongs to the TCF/Sox gene family since more than 90% identity is observed at the protein level with five HMGbox regions from various species (Figure 1). Such a high homology between these genes suggests that these sequences may be considered as orthologues. Furthermore, 66% and 80% identity is observed when the nucleic acid sequence of the new gene is compared to those of SRY and SOX-9 respectively, excluding the hypothesis of sequence convergence. The amino acid positions 39 (W) and 47 (K) in Pw Sox-1 also appear to be very conserved in HMG-box superfamily (8). Southern analysis using PwSox-1 as a probe (not shown) also reveals the presence of similar sequences in another Urodele Amphibian. Experiments in lower vertebrates have not thus revealed for any sex chromosome-linked Sox gene homologuous to SRY in mammals (7, 9), perhaps because X:Y and Z:W sex determining systems have separately evolved (10). In Pleurodeles, a species with a female ZW heterogamety, PwSox-1 was detected by RNA—RNA in situ hybridization on meiotic lampbrush autosome VI (unpublished results), meaning that PwSox-1 is transcribed during oogenesis. Since Sox genes are suspected to play a role in development and differentiation mechanisms, and since PwSox-1 appears to belong to TCF/Sox family that contains transcription factors, it may be hypothesized that this new Sox gene is involved in early development. ACKNOWLEDGEMENTS