A FUNCTIONAL POLYADENYLATION SIGNAL IS EMBEDDED IN THE CODING REGION OF CHICKEN GROWTH-HORMONE RECEPTOR RNA

A study of chicken GH receptor (cGHR) expression has revealed that the two major liver and skeletal muscle transcripts of the cGHR are devel opmentally expressed. Expression of the larger (4.7 kilobases) transcr ipt increases with age. The smaller transcript (0.7 kilobases) is a tr uncation product, resulting from alternative usage of a functional pol yadenylation [poly(A)] signal embedded in the coding sequence. The ext ent to which alternative cleavage and polyadenylation occur displays s ome tissue and sex specificity. Cleavage and polyadenylation occur dow n-stream of the AATAAA portion of the poly(A) signal (cGHR positions 3 04-309) and up-stream of a GT-rich sequence. The truncated transcript appears to be translated, based on its association in vivo with polyri bosomes, although the physiological role of the putative protein produ ct of this truncated transcript is as yet unknown. Three other avian s pecies (quail, turkey, and duck) also show a polyadenylated truncation of the GHR message due to a poly(A) signal at the same location in th e coding sequence. In cell culture expression, mutation of AATAAA to A ACAAG prevents production of the truncated transcript. In a chimeric c onstruct, the signal and neighboring sequence from the cGHR are suffic ient to confer cleavage and polyadenylation upon the rat GHR, a gene t hat otherwise lacks the internal poly(A) signal. Alternative polyadeny lation within the coding region of a structural gene is discussed as a heretofore unknown means of posttranscriptional regulation of a gene product.