STRUCTURE, EVOLUTION AND CHROMOSOMAL LOCALIZATION OF THE HUMAN PREGNANCY-SPECIFIC-BETA-1 GLYCOPROTEIN GENE FAMILY

Cloning and sequencing of the cDNA of a new member of the PSβG gene family is reported. Comparison of the sequence with those of other PSβG cDNAs reveals a remarkable conservation of their sequence (>90%) and of their general structural organization: an NH2 domain is followed by 93- and 85-residue Ig-like domains termed A and B, respectively. Most PSβG contain two domains A in tandem followed by a single domain B. In some PSβG members described here, alternative splicing skips the AI domain in some transcripts, yielding two- and three-domain variants, respectively. Individual PSβG members have specific short carboxyl domains displaying little sequence conservation. The PSβG family is closely related to the CEA gene family. A detailed comparison of the sequence of both families is given and used to construct an evolutionary tree, using the method of Li, Wu, and Luo (1985, Mol. Biol. Evol. 2: 150-174). Computation of the number of substitutions of synonymous (Ks) and nonsynonymous (Ka) sites and of the Ks/Ka ratio suggests that the PSβG gene family appeared concomitantly with the expansion of the placental mammals and belongs to the class of rapidly evolving genes. Very little selective pressure has been exerted on the body of the molecules, especially on domain A. The analysis also suggests that PSβG genes encoding different carboxyl domains would have been positively selected and fixed during the evolution. The PSβG gene family was assigned to chromosome 19, which also carries the CEA genes. © 1990.