Sequence and expression of hamster prolactin and growth hormone messenger RNAs

Complementary DNAs encompassing the complete protein-encoding regions for PRL and GH of the Syrian Golden hamster were sequenced and used as probes to examine the expression of hamster PRL and GH messenger RNA (mRNA)s. The complementary DNA (cDNA) for hamster PRL encodes a 226 amino acid preprotein which, by analogy to rat and mouse PRLs, is predicted to be processed to yield a 197 amino acid secreted protein. The hamster GH cDNA codes for a 216 amino acid preprotein predicted to yield a 190 amino acid secreted protein. Both hamster proteins are highly homologous to the corresponding rat and mouse hormones. For the secreted proteins, hamster PRL has 82% amino acid identity with rat PRL and 72% identity with mouse PRL. The rodent GH sequences are more strongly conserved, with 97-98% sequence identity between hamster, rat, and mouse GHs. The hamster hormones contain the highly conserved cysteine residues (six in hamster PRL and four in hamster GH) present in other mammalian PRLs and GHs. Neither hamster PRL nor hamster GH contains cysteine residues corresponding to the unique pair of cysteines present in hamster placental lactogen-II. The hamster PRL and GH cDNAs each hybridized to pituitary mRNAs of approximately 1 kilobase. Expression of hamster PRL and GH mRNAs was compared between 2 days of the estrous cycle (proestrus and estrus) and early, mid, and late pregnancy (days 5, 10, and 15). PRL mRNA levels in cycling hamsters were approximately 50% of those in pregnant hamsters. No other significant differences in PRL or GH mRNA levels were observed, suggesting that differences in circulating PRL and GH protein levels during the estrous cycle and pregnancy in the hamster are the result largely of factors other than changes in mRNA levels.