M. Jakubowski et Jl. Roberts, PROCESSING OF GONADOTROPIN-RELEASING-HORMONE GENE TRANSCRIPTS IN THE RAT-BRAIN, The Journal of biological chemistry, 269(6), 1994, pp. 4078-4083
The precursor of gonadotropin-releasing hormone (GnRH) and the 56-amin
o acid GnRH-associated peptide is encoded in an mRNA of about 560 base
s in length. This mRNA derives from an similar to-4300-base pair-long
gene consisting of four relatively short exons (denoted 1, 2, 3, and 4
) and three large introns (A,B, and C). In this study, we characterize
d the order by which the three introns are spliced from the primary tr
anscript and processing intermediates to give rise to a mature mRNA an
d evaluated the potential role of gene transcription and pre-mRNA proc
essing in the control of proGnRH mRNA levels in vivo. Nuclear and cyto
plasmic RNA fractions isolated from rat preoptic area anterior hypotha
lamus (POA-AH) and basal olfactory area (located rostral to the POA) w
ere analyzed by 1) solution hybridization-RNase protection mapping usi
ng several RNA probes directed at various regions of the proGnRH gene
and 2) reverse transcription-polymerase chain reaction using several o
ligonucleotide primers. Both types of analysis showed that proGnRH pre
-mRNA processing begins with the splicing of intron B from the primary
gene transcript. Hence, intron B is the ideal target for studying pro
GnRH primary transcript by in situ hybridization. Subsequent splicing
of introns A and C appeared to take place in two alternative, although
not equally prevalent pathways. Quantitative analysis indicated that
the proGnRH hnRNA species constituted, ore a mole basis, about 20% of
the total gene transcripts in the POA-AH. The primary transcript alone
constituted about 10% of the total gene transcripts in the POA-AH and
as much as 20% in the basal olfactory area. The prospect of blockade
of proGnRH hnRNA processing by means of hybridization with endogenous
antisense RNAs (transcribed from the SH gene on the opposite strand of
the same DNA locus) did not prove to be likely, as the SH transcripts
were present at very low levels compared to any of the proGnRH RNA sp
ecies. We conclude that the relatively large pool of proGnRH hnRNA may
reflect a high rate of gene transcription and/or slow RNA processing.