Generate signature within XML payload in SOAPUI

Question

I have the following XML and need to generate a signature instead of the marker "{$SIGNATURE}", the signature should be RSA-SHA1 of the content inside the "content" tag.

<sig digestMethod="SHA1" sigMethod="RSA-SHA1" thumbprint="XXXXXX">
{$SIGNATURE}
</sig>
<content>
  <app-id>yxz123</app-id>
  <hmac>HMACSHA256</hmac>
  <signing-time>${=new java.text.Simpl${HTTP Request#ResponseAsXml}eDateFormat("yyyy-MM-dd'T'HH:mm:ss").format(new Date())}
  </signing-time>
</content>