1. RNA isolated from the post-nuclear supernatant of the lactating guinea-pig mammary gland was fractionated with oligo(dT)–cellulose into three populations; those that bound at ‘low salt’ [long poly(A) tracts, 78–32 nucleotides]; those that bound at ‘high salt’ [shorter poly(A) tracts, 48–21 nucleotides]; and those that did not bind [no poly(A) or short poly(A) tracts, <20 nucleotides]. Nuclear RNA was fractionated into two populations, those that bound in ‘low salt’ and those that did not bind. All the post-nuclear RNA fractions directed the synthesis of milk proteins in a Krebs II ascites cell-free system. 2. 3H-labelled DNA complementary to the post-nuclear poly-(A)-containing RNA population (low-salt fraction) was fractionated into abundant (milk-protein mRNA), moderately abundant and scarce sequences. This complementary DNA was then used to investigate the distribution of the mRNA sequences in the different RNA populations. This showed that all sequences were present in polyadenylated and non-polyadenylated fractions, but that major quantitative differences were apparent. The abundant milk-protein mRNA sequences predominated in the ‘low-salt’ post-nuclear poly(A)-containing RNA fraction, whereas the moderately abundant sequences predominated in the non-polyadenylated post-nuclear RNA fraction. In total cellular RNA, those sequences deemed initially to be moderately abundant within the ‘low-salt’ poly(A)-containing RNA population were present at a concentration very similar to those of the abundant milk-protein mRNA (approx. 6×105 copies of each sequence/cell). Similarly, analysis of the nuclear RNA populations showed that the ‘abundant’ and so-called ‘moderately abundant’ sequences were present in essentially identical concentrations (2×103 copies of each sequence/cell). The majority of these (90–95%) were non-polyadenylated. 3. The results are discussed in terms of the post-transcriptional mechanisms involved in the regulation of gene expression in the lactating guinea-pig mammary gland.
Esterification of glycerol 3-phosphate in lactating guinea-pig mammary gland