RECENT EVOLUTION OF GENES ENCODING THE PROHORMONE-LIKE PROTEIN SMR1 IN THE RAT SUBMANDIBULAR-GLAND

The Variable Coding Sequence (VCS) multigene family of Rattus norvegic us, is composed of at least 10 members, and shows extensive evolutiona ry divergence in the protein-coding region. Three members of the VCSA subclass, have been characterized: one of them, the VCSA1 gene mainly expressed in the submandibular gland (SMG) encodes the prohormone-like protein, SMR1-VA1. As VCSA-related genes have not been detected in Mu s musculus, the VCSA genes subclass-is presumed to have recently emerg ed. To study the evolution of this subclass, we have looked for VCSA g enes in a closely related species, Rattus rattus. By Northern analysis , we demonstrate that VCS-related mRNAs are present in the SMG, and th at the level of VCSA mRNA accumulation is approximately equal in both sexes. By contrast, in R, norvegicus, males accumulate about 3,000 tim es more VCSA1 mRNA than females. Using total SMG mRNA, an almost full- length cDNA, homologous to the cDNA of the R, norvegicus VCSA1 gene, w as cloned by reverse transcriptase polymerase chain reaction (RT-PCR). The putative corresponding SMR1-VA1 protein is 146 amino acids long a nd presents the features characteristic of a secreted protein, with a potential signal peptide of 22 amino acids in the amino-terminal porti on. The presence of potential processing multibasic sites suggests tha t small peptides could be generated (particularly a hexapeptide: Arg-G ln-His-Asn-Leu-Arg), as in the case of the SMR1-VA1 protein of R, norv egicus. From Southern blot analysis there appears that species-species modifications of VCSA gene copy number have occurred; R, rattus conta ins a greater VCSA1 copy number than R, norvegicus (two or three and o ne, respectively).