Question: Who was the emperor while Prince Sanjō Sanetomi was an Imperial court noble? 

Prince Yamashiro (山背大兄王 , Yamashiro no Ōe no Ō , died 643) was the eldest son of one of the most famous figures in Japanese history, Prince Shōtoku.  Yamashiro claimed the right to Imperial succession in 628, following the death of Empress Suiko.  However, he lost the claim to Prince Tamura who ascended to the throne as Emperor Jomei, having enjoyed the support of powerful court noble Soga no Emishi. <SEP>  Although there were Emperors before the Meiji Restoration, the events restored practical abilities and consolidated the political system under the Emperor of Japan. The Meiji Restoration (明治維新 , Meiji Ishin ) , also known as the "Meiji Ishin", "Renovation", "Revolution", "Reform", or "Renewal", was an event that restored practical imperial rule to Japan in 1868 under Emperor Meiji. <SEP>  He held many high-ranking offices in the Meiji government. Prince Sanjō Sanetomi (三条 実美 , 13 March 1837 – 18 February 1891) was an Imperial court noble and statesman at the time of the Meiji Restoration. <SEP> Fanzhen (: "fānzhèn" ) was a governmental system involving administration through regional governors ("jiedushi)".  The term "fanzhen" literally means "buffer town", and refers to the strategic settlement of troops in locations along the empire's border areas.  During the Tang dynasty these settlements came under the control of provincial military commissioners, otherwise known as "jiedushi".  As control of these "fanzhen" devolved from central authority into the hands of the local leaders, they at times became powerful enough to threaten the imperial court (618–907 CE), particularly during and after the An Lushan Rebellion.  An Lushan, the provincial governor and military commander who started this rebellion against Emperor Xuanzong of Tang, in 755 CE, went so far as to proclaim himself emperor, in 756 CE, but, was killed by his own son in the following year and Tang power was re-established by 763 CE.  The An Lushan Rebellion allowed many "jiedushi" on the periphery of the Tang Empire to gain significant autonomy with many becoming warlords in all but name.  Subsequent Tang emperors were met with lukewarm success in curtailing the power of these "fanzhen", in particular, Emperor Dezong of Tang (r. 779–805 CE) who was driven from his capital, Chang'an, after an unsuccessful attempt to subjugate them.  Subsequent Emperor Xianzong of Tang (r. 805–820 CE) was able to suppress some "fanzhen" but at the cost of further empowering the eunuchs who had come to dominate the life of the Imperial Court.  Xianzong died in 820 CE, possibly as a result of assassination, and his successors were unable to stop the dynasty's decline.  The ambitions of the "jiedushi", in tandem with the corruption of the Imperial Court eunuchs who dominated the central civil administration and even attained high military command under the later Tang, contributed to the disintegration of the Tang monarchy.  A brief resurgence under emperors Wuzong and Xuānzong failed to halt the decentralization of state power, and the Tang collapsed following a further series of major peasant uprisings like the Wang Xianzhi and Huang Chao rebellions.