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Gene expression in oviparous vertebrates during vitellogenesis is hormonally regulated. Our laboratory has characterized a unique gene (wf female), which is seasonally expressed in the liver of the female winter flounder Pseudopleuronectes americanus. The wf female mRNA is coexpressed with vitellogenin mRNA and reaches a high level during vitellogenesis. The wf female gene is 2554 base pairs in length and encodes a putative protein of 509 amino acids. The gene consists of eight exons separated by seven introns of different sizes. Within exon 1, there are six PQQ-rich repeats. Four of them encode a putative (PQQ)1PKY polypeptide, similar to the repeats found in the extracellular domains of other proteins. Exons 2-7 share homology with the zona pellucida protein genes rc55 of rabbit and zp-2 of mouse, and the positions of intron boundaries are conserved in the wf female and mouse zp-2 genes. In addition, the transcriptional regulatory cis elements (estrogen response element, CCAAT and TATAAA boxes), as found in vertebrate genes, are also conserved for the wf female promoter and mapped upstream from the initiation site of the wf female primary transcript.

The seasonal cycle and regulation by temperature of antifreeze protein mRNA (AF mRNA) were investigated in a Long Island population of winter flounder (Pseudopleuronectes americanus) by Northern blot hybridization and by in vitro translation of liver RNA. AF mRNA was expressed at high levels in the fall and winter (Nov.-Feb.) and at low or undetectable levels in the summer. The time of accumulation of AF mRNA coincides with the time during which water temperature and photoperiod decrease to 4°C and 9 h of light per day, respectively. A temperature and photoperiod decrease in the laboratory during this time also resulted in high levels of AF mRNA. The levels of other mRNAs, as assayed by in vitro translation, were relatively constant during both seasonal acclimation and laboratory acclimation. The seasonal cycle of AF mRNA in Long Island winter flounder is similar to that of a more northern, Newfoundland population of winter flounder and different from that of an intermediate, New Brunswick population. These similarities and dissimilarities are discussed in light of potentially different exogenous and endogenous regulatory cues in the different populations.