Two temporal phases for the control of histone gene activity in cleaving sea urchin embryos (S. purpuratus)☆

[摘要] Thisreportpresentsananalysisofhistonegeneexpressioninthecleavingembryooftheseaurchin,Strongylocentrotuspurpuratus,withemphasisonwhethertheregulatorysite(s)inthepathwayofgeneexpressionchangeasdevelopmentproceeds.Theanalysisfocusesontheequation,dP∗dt=M·f·n·At,wheredP∗dt=theabsoluterateofhistonesynthesis;M=themolequantityofhistonemessengerRNA;f=thefractionofhistonemRNAinpolysomes;n=thepolysomesize;andAt=therateofelongationofnascenthistonepolypeptidechains.Theembryosolvesthisrateequationdifferentlyatdifferenttimes.Measurementsweremade(at15°C)ofabsoluteratesofhistonesynthesis(dP∗dt).Therateofhistonesynthesisincreasesatleast48-foldduringthefirst6hrafterfertilizationfromlessthan0.5to24pgembryo−1hr−1;intheperiodfrom6to12hr,thisraterisesto182pgembryo−1hr−1,anadditional7.7-foldrise,resultinginanoverallincreaseof370-foldbetweenthe1-celland200-cellstage.Thefractionofnewlysynthesized(zygotic)histonemessengerRNAthatpartitionsintopolysomes(fzygotic)hasalsobeenmeasuredduringthefirst12hrofdevelopment.Thisfractionincreasesfrom0.2inthe2-hrembryoto0.8inthe6-hrembryo(16-cellstage),increasingslowlythereaftertonearunityby12hr.Thesizeofhistone-synthesizingpolysomes(n)doesnotchangesubstantiallyoverthe12-hrinterval,remainingconstantataweightedmeanof5ribosomesperpolysome(range3to7).UtilizingthedataonfzygoticanddP∗dt,therateofelongationofnascenthistonepolypeptidechains(At)duringthefirst6hrofdevelopmentwasestimated;Atremainsconstantat1.11codonspersecond.Thiscalculatedvalueisinfairagreementwithadirectmeasurementofhistonepeptideelongationrateinthe12-hrembryo.Itisproposedthathistonegeneexpressionincleavingseaurchinembryosbedividedintotwophases,distinguishedonthebasisoftheirpivotaltranslationalparameters:PhaseI(0–6hr),duringwhichfisratedetermining,andPhaseII(6hron),duringwhichMistherate-determiningparameter.

[学科分类] 生物科学（综合）