Over 40 years ago, Allfrey and colleagues (1964) suggested that two histone modifications, namely acetylation and methylation, might regulate RNA synthesis. Nowadays it is universally accepted that activation of gene expression strictly depends on enzymatic mechanisms able to dynamically modify chromatin structure. Here, using techniques including DNaseI hypersensitive site analysis, chomatin immunoprecipitation and quantitative PCR analysis, we have analyzed the dynamics of histone post-translation modifications involved in developmentally/spatially controlled activation of the sea urchin PlTalpha2 tubulin gene. We have demonstrated that only when the PlTalpha2 core promoter chromatin is acetylated on H3K9, tri-methylated on H3K4 and not di-methylated on H3K27, RNA pol II can be enrolled. In contrast, we have shown that when chromatin is methylated both on H3K9 (me2/3) and H3K27 (me2) and mono methylated on H3K4 the promoter is not accessible to RNA pol II. Our results suggest that, during P. lividus embryogenesis, both HAT/HDAC and HMT/HDM activities, which are able to regulate accessibility of the PlTalpha2 basal promoter to RNA polymerase II, are coordinately switched-on.