[laura.g.berger]

Hello!
Guys, could you, please, tell me what does means, in the attached picture(phosphate group), that oxygen with minus signal(O-)?

Is there a hydrogen there and it's omitted in this picture?

Thank you!

[Yggdrasil]

The O^- sign means an oxygen with an overall negative charge.  This happens when the hydrogen gets removed from the phosphate group, leaving a lone pair of electrons.

[laura.g.berger]

The O- sign means an oxygen with an overall negative charge.  This happens when the hydrogen gets removed from the phosphate group, leaving a lone pair of electrons.

Thank you for your answer!

It is possible i replace these oxygen for other atom of other chemical element? Or both oxygens (that with double bond with phosphate)? If yes, which element could be?

[Yggdrasil]

It is possible to replace one of the oxygens with another element.  Replacing one of the oxygen in the phosphate results in a methylphosphonate group (see http://www.trilinkbiotech.com/cart/scripts/prodView.asp?idproduct=3305) which lacks a negative charge.  Replacing the oxygen with a sulfur creates a phosphothioate group (see http://www.trilinkbiotech.com/cart/scripts/prodView.asp?idproduct=3302) which makes the phosphate less reactive and less prone to degradation.