Synthesis of the mouse myelin basic proteins (MBPs) ular masses of the principal variants are approximately was studied in reticulocyte lysates programmed with 21.5 kD, 18.5 kD, 17 kD, and 14 kD on SDS-polyacrylbrain mRNA in the presence or absence of brain amide gels. We have recently shown that there are two factors. Addition of brain factors to the lysates informs of the mouse 17-kD variant, the molecular masses creased the incorporation of [35S]methionine into total of which are within 100 daltons of each other and which TCA-precipitable protein by a factor of 6-9, and the are probably not resolved by SDS-PAGE (Newman, Kimajority of this stimulation was found to be due to tamura, and Campagnoni, in press). The mRNAs for the initiation factors. Although brain factors increased MBPs appear to be derived from a single gene by differtotal protein synthesis, the percentage of MBP syntheential splicing sis was reduced from 4.3% of the total counts incor-deFerra et al, 19851 and their expression appears to be porated (in the absence of brain factors) to 1.4% (in developmentally regulated with the 14-kD MBP increastheir presence). Increasing the concentration of brain ing relative to the others throughout early postnatal de-mRNA in the lysates also reduced the relative levels velopment Carson et al, 19831. of MBP synthesis. These results suggested the MBP Regulation at the translational level may be an im-mRNAs, as a group, were less efficiently initiated than portant mechanism for controlling gene expression in most brain mRNAs. An analysis of the nucleotide eukaryotes Moldave, 19851. Our own labsequence flanking the initiator codon of the MBP oratory has been interested in the expression of myelin mRNAs indicates the presence of a second AUG codon proteins and we have extensively examined their synthe-5 bases upstream, immediately followed by a termisis in vivo [Campagnoni et al, 19781 and in a number of nation codon, which may provide a structural explacell-free, in vitro systems, including brain homogenates nation for the poorer initiation efficiency of the MBP [Carey and Campagnoni, 19791, brain polyribosome mRNAs. Further analysis of the synthesis of the indi-"run-off" systems [Yu and Campagnoni, 19821, and vidual MBPs in the presence or absence of inhibitors mRNA-stimulated reticulocyte lysates [Roth et al, 19851 of initiation (7-methylguanosine triphosphate and auand wheat germ extracts [Matthees and Campagnoni, rintricarboxylic acid) and elongation (anisomycin and 19801. One observation to emerge from these studies has emetine) indicated that the 14-kD MBP mRNA was been that the levels of MBP synthesis compared to total less efficiently translated than the other MBP mRNAs. incorporation and the relative proportions of the individ-Synthesis of the 14-kD MBP was more strongly inhibual mouse MBPs synthesized differed considerably ited by the initiation inhibitors than the other MBP among the systems examined. The purpose of this study mRNAs, and synthesis of the 14-kD MBP was inwas to investigate the causes for the lack of consistency creased relative to the other MBPs in the presence of in the observed levels of MBP synthesis and the proporelongation inhibitors. These results are consistent with tions of individual MBPs synthesized in these systems. the notion that the 14-kD MBP mRNA is initiated less An additional purpose was to determine if the mouse efficiently than the other MBP mRNAs.
MBP mRNAs were translated with different efficiencies K~~ words: protein synthesis initiation, translation, and what might be the reason for this. In this article we show that the MBP mRNAs are not as efficiently trans-mRNA