GONADOTROPIN ALPHA-SUBUNIT GLYCOPROTEIN FROM CHANNEL CATFISH (ICTALURUS-PUNCTATUS) AND ITS EXPRESSION DURING HORMONE-INDUCED OVULATION

Complementary DNA (cDNA) encoding the channel catfish (Ictalurus punct atus) gonadotropin (GTH) alpha-subunit glycoprotein was cloned by poly merase chain reaction (PCR) from a plasmid library made from pituitary RNA. Complete cDNA cloning was achieved by carrying out two PCP react ions: one with an upstream sense primer plus the universal sequencing primer, located downstream of the poly(A) sequence of the cDNA in the plasmid vector, to amplify the downstream portion of the cDNA; the oth er with a downstream antisense primer plus the reverse-sequencing prim er, located upstream of the very 5' end of the cDNA sense strand in th e plasmid vector, to amplify the upstream portion of the cDNA. The two amplified fragments overlapped about 70 bp. Nucleotide sequence analy sis revealed that the catfish GTH alpha-subunit was 658 bp encoding 11 6 amino acids and harboring a 5' nontranslated region (NTR) of 42 bp a nd a 3' NTR of 265 bp. The deduced amino acid sequence of the catfish GTH alpha-subunit is highly conserved with those from other cloned tel eost GTH alpha-subunits. The GTH alpha-subunit was highly expressed ev en before induction for ovulation in females during spawning season. A dministration of carp pituitary extract (a spawning-inducing reagent) induced only 1.4-fold higher expression of the GTH alpha-subunit RNA, but induced very rapid egg maturation and ovulation. This unexpected r esult indicated that the GTH alpha-subunit may not be the limiting fac tor for ovulation and spawning, which may be regulated by a change of propertional coupling of the GTH alpha-subunit with specific beta-subu nits during hormone-induced ovulation.