Genetic factors responsible for the mobility of mPing in the rice genome

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In a mutant line IM294 (slender glume mutant) which harbors Rurm1 (Rice Ubiquitine Related Modifier-1) allele inactivated by mPing insertion shows high excision frequency (ca.1%) of mPing from mutant Rurm1 allele (Rurm1^m). Expression levels of two ORFs of Ping were also high in IM294 comparing to the original variety Gimbozu. To clarify the relation between the function of Rurm1 and Ping activity under the same genetic background, the mutant allele of Rurm1 was introduced into Gimbozu background through recurrent backcrossing. Also, a pair of non-slender glume line (Rurm1^+/Rrum1^+) and slender glume line (Rurm1^m/Rurm1^m) derived from offspring of the non-slender glume plant (Rurm1^+/Rurm1^m) segregated among IM294 line were used. Comparing the Gimbozu and its isogenic line and sib lines, inactivation of Rurm1 with mPing insertion enhanced the expression levels of two ORFs of Ping. On the other hand, significant change in the expression level of Pong was not observed. These indicate that the inactivation of Rurm1 promotes the mPing excision by enhancing the expression of Ping's ORFs. Though, the inactivation of transposon is generally regulated by methylation, it is quite unlikely that RURM1 protein directly regulate the methylation of DNA. Effect of the inactivation of Rurm1 allele was also analyzed using 22k-micro array (Agilent Co.,). Inactivation of Rurm1 upregulate the expression of protein biosynthesis related genes in ribosome and nucleus. It also downregulated the expression of genes related to metabolism and physiological process. These indicate that the function of Rurm1 is related to transcription of vast number of genes related to the physiological process. Upregulation of protein synthesis related genes indicates the compensation to growth retardation of plants. Upregulation of genes related to DNA-repair process was also observed indicating the frequent DNA double strands break induced by mPing excision in IM294.