Protein synthesis by gastric mucosal ribosomes during development in rats

Oxyntic gland mucosal ribosomes from 7- to 30-day-old rats were assayed for their ability to synthesize endogenous messenger RNA (mRNA) and poly(U)-directed protein in a cell-free system. Normal rat liver cell sap (105,000 g supernatant) was used as a source for transfer RNA (tRNA) and activating enzymes. Total mucosal DNA and ribosome content [as assessed by ribosomal RNA (rRNA)] were also determined. Although both DNA and rRNA content increased throughout the 4 postnatal weeks, the rRNA/DNA ratio rose sharply during the 2nd and 4th week after birth. The ability of mucosal polyribosomes to synthesize endogenous mRNA-directed protein in a cell-free system remained elevated up to the age of 18 days, then decreased markedly within the next 3 days, and thereafter, declined more slowly. In 21- and 30-day-old rats, the rate of endogenous mRNA-directed protein synthesis by mucosal ribosomes was 50-60% below that of the 7-day-old suckling rats. The protein/polyphenylalanine ratio, which represents a ratio of polysomes to monosomes, was found to be about 57% lower for ribosomes from 30-day-old rats as compared to 7-day-old suckling animals. The ability of run-off ribosomes (devoid of endogenous mRNA) to translate poly(U) was also found to be 33% lower for 30-day-old rats as compared to the 7-day-old suckling animals.