CHANGES IN RIBOSOMAL-PROTEIN AND RIBOSOMAL-RNA SYNTHESIS DURING RAT INTESTINAL DIFFERENTIATION

Subtraction hybridization studies, used to identify genes involved in the control of enterocyte proliferation and/or differentiation, allowe d detection of a clone shown to have homologies with rat, chicken, and human acidic ribosomal phosphoprotein P1. Since increases in P1 trans cript have been associated with intestinal malignancy, we explored the relationship of P1 and other ribosomal proteins to normal intestinal proliferation and differentiation. Male rats were used to prepare ente rocytes as isolated cell fractions representative of the crypt to vill us axis of differentiation. Total RNA was extracted from pooled cell f ractions and evaluated for mRNA and rRNA steady-state levels. Nuclei w ere prepared from isolated enterocytes, and nuclear runoff studies wer e performed to estimate rates of nascent transcription. The P1 complem entary DNA from the crypt cell library detected a mRNA of 650 base pai rs which showed approximately 8-fold greater steady-state levels in cr ypt than in villus cells. Similar crypt specificity was also noted for mRNAs coding for elongation factor EF-12 and for ribosomal proteins P 0, P1, and S6 (using clones from Y-L. Chan and I. G. Wool). In contras t, 28S rRNA steady-state levels did not differ between villus and cryp t, indicating that ribosomal content had remained constant. In situ hy bridization studies confirmed the predominant crypt localization of P1 mRNA. Nascent transcription rate studies showed that the proportion o f newly synthesized P1 mRNA to total RNA was the same for the villus a nd crypt, suggesting that the lower content of villus P1 mRNA may be d ue to increased degradation. These results suggest that the high rate of ribosomal phosphoprotein synthesis during enterocyte proliferation is markedly decreased upon completion of differentiation. Changes in r ibosomal phosphoprotein synthesis may be a determinant of irreversible differentiation.